Winlogon.exe process thrashing with McAfee real-time scanning - malware infection?

Very recently, this Dell PC running XP Home (sp3, with all updates current) and McAfee (VirusScan build 15.0.288. engine version 5400.1158, DAT version 6375 of 6/13/2011) begain experiencing very high CPU spikes and slow performance.  The winlogon.exe process was spiking from 10% to 80% about once every second, and web searches suggested this had in the past been a symptom of a malware infection.  McAfee full scans revealed nothing. However, the CPU activity would return to normal when McAfee real-time scanning is turned off, and will spike again when real-time protection is re-enabled.  McAfee's only suggestion was to reinstall McAfee (done, problem persists) so it's time to explore other options.

Thanks in advance for your help.

HIJACKTHIS LOG FOLLOWS:

Logfile of Trend Micro HijackThis v2.0.4
Scan saved at 8:16:10 PM, on 6/14/2011
Platform: Windows XP SP3 (WinNT 5.01.2600)
MSIE: Internet Explorer v8.00 (8.00.6001.18702)
Boot mode: Normal

Running processes:
C:\WINDOWS\System32\smss.exe
C:\WINDOWS\system32\winlogon.exe
C:\WINDOWS\system32\services.exe
C:\WINDOWS\system32\lsass.exe
C:\WINDOWS\system32\svchost.exe
C:\WINDOWS\System32\svchost.exe
C:\WINDOWS\system32\spoolsv.exe
C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\mscorsvw.exe
C:\Program Files\Java\jre6\bin\jqs.exe
C:\Program Files\Common Files\McAfee\McSvcHost\McSvHost.exe
C:\Program Files\Common Files\McAfee\SystemCore\mfevtps.exe
C:\WINDOWS\System32\svchost.exe
C:\Program Files\Common Files\McAfee\SystemCore\mcshield.exe
C:\Program Files\Common Files\McAfee\SystemCore\mfefire.exe
C:\WINDOWS\Explorer.EXE
C:\Program Files\McAfee.com\Agent\mcagent.exe
C:\WINDOWS\system32\ctfmon.exe
C:\WINDOWS\system32\taskmgr.exe
C:\Program Files\Internet Explorer\iexplore.exe
C:\Program Files\Internet Explorer\iexplore.exe
C:\Program Files\SiteAdvisor\6253\SiteAdv.exe
C:\Program Files\Internet Explorer\iexplore.exe
C:\Documents and Settings\USER02\Desktop\HijackThis.exe

R1 - HKCU\Software\Microsoft\Internet Explorer\Main,Default_Page_URL = http://www.yahoo.com/?fr=fp-yie8
R0 - HKCU\Software\Microsoft\Internet Explorer\Main,Start Page = http://us.f409.mail.yahoo.com/ym/login
R1 - HKLM\Software\Microsoft\Internet Explorer\Main,Default_Page_URL = http://www.dell4me.com/myway
R1 - HKLM\Software\Microsoft\Internet Explorer\Main,Search Bar = http://red.clientapps.yahoo.com/customize/ie/defaults/sb/ymsgr/*http://www.yahoo.com/ext/search/search.html
R0 - HKLM\Software\Microsoft\Internet Explorer\Main,Start Page = http://go.microsoft.com/fwlink/?LinkId=69157
R1 - HKCU\Software\Microsoft\Internet Explorer\Main,Window Title = Windows Internet Explorer provided by Yahoo!
R3 - URLSearchHook: (no name) - {EF99BD32-C1FB-11D2-892F-0090271D4F88} - (no file)
O2 - BHO: (no name) - {02478D38-C3F9-4EFB-9B51-7695ECA05670} - (no file)
O2 - BHO: Adobe PDF Reader Link Helper - {06849E9F-C8D7-4D59-B87D-784B7D6BE0B3} - C:\Program Files\Adobe\Acrobat 7.0\ActiveX\AcroIEHelper.dll (file missing)
O2 - BHO: (no name) - {089FD14D-132B-48FC-8861-0048AE113215} - C:\Program Files\SiteAdvisor\6253\SiteAdv.dll
O2 - BHO: McAfee Phishing Filter - {27B4851A-3207-45A2-B947-BE8AFE6163AB} - c:\progra~1\mcafee\msk\mskapbho.dll (file missing)
O2 - BHO: scriptproxy - {7DB2D5A0-7241-4E79-B68D-6309F01C5231} - C:\Program Files\Common Files\McAfee\SystemCore\ScriptSn.20110531175322.dll
O2 - BHO: Windows Live Sign-in Helper - {9030D464-4C02-4ABF-8ECC-5164760863C6} - C:\Program Files\Common Files\Microsoft Shared\Windows Live\WindowsLiveLogin.dll
O2 - BHO: Java(tm) Plug-In 2 SSV Helper - {DBC80044-A445-435b-BC74-9C25C1C588A9} - C:\Program Files\Java\jre6\bin\jp2ssv.dll
O2 - BHO: JQSIEStartDetectorImpl - {E7E6F031-17CE-4C07-BC86-EABFE594F69C} - C:\Program Files\Java\jre6\lib\deploy\jqs\ie\jqs_plugin.dll
O3 - Toolbar: McAfee SiteAdvisor - {0BF43445-2F28-4351-9252-17FE6E806AA0} - C:\Program Files\SiteAdvisor\6253\SiteAdv.dll
O4 - HKLM\..\Run: [IgfxTray] C:\WINDOWS\system32\igfxtray.exe
O4 - HKLM\..\Run: [Lexmark 5200 series] "C:\Program Files\Lexmark 5200 series\lxbtbmgr.exe"
O4 - HKLM\..\Run: [LXBTCATS] rundll32 C:\WINDOWS\System32\spool\DRIVERS\W32X86\3\LXBTtime.dll,_RunDLLEntry@16
O4 - HKLM\..\Run: [LVCOMS] C:\Program Files\Common Files\Logitech\QCDriver2\LVCOMS.EXE
O4 - HKLM\..\Run: [UserFaultCheck] %systemroot%\system32\dumprep 0 -u
O4 - HKLM\..\Run: [SiteAdvisor] C:\Program Files\SiteAdvisor\6172\SiteAdv.exe
O4 - HKLM\..\Run: [REGSHAVE] C:\Program Files\REGSHAVE\REGSHAVE.EXE /AUTORUN
O4 - HKLM\..\Run: [mcui_exe] "C:\Program Files\McAfee.com\Agent\mcagent.exe" /runkey
O4 - HKLM\..\Run: [SunJavaUpdateSched] "C:\Program Files\Common Files\Java\Java Update\jusched.exe"
O4 - HKCU\..\Run: [MsnMsgr] "C:\Program Files\MSN Messenger\MsnMsgr.Exe" /background
O4 - HKCU\..\Run: [DellSupportCenter] "C:\Program Files\Dell Support Center\bin\sprtcmd.exe" /P DellSupportCenter
O4 - HKCU\..\Run: [ctfmon.exe] C:\WINDOWS\system32\ctfmon.exe
O4 - HKCU\..\Run: [Search Protection] C:\Program Files\Yahoo!\Search Protection\SearchProtection.exe
O9 - Extra button: (no name) - {08B0E5C0-4FCB-11CF-AAA5-00401C608501} - C:\Program Files\Java\jre6\bin\jp2iexp.dll
O9 - Extra 'Tools' menuitem: Sun Java Console - {08B0E5C0-4FCB-11CF-AAA5-00401C608501} - C:\Program Files\Java\jre6\bin\jp2iexp.dll
O9 - Extra button: (no name) - {e2e2dd38-d088-4134-82b7-f2ba38496583} - C:\WINDOWS\Network Diagnostic\xpnetdiag.exe
O9 - Extra 'Tools' menuitem: @xpsp3res.dll,-20001 - {e2e2dd38-d088-4134-82b7-f2ba38496583} - C:\WINDOWS\Network Diagnostic\xpnetdiag.exe
O9 - Extra button: Messenger - {FB5F1910-F110-11d2-BB9E-00C04F795683} - C:\Program Files\Messenger\msmsgs.exe
O9 - Extra 'Tools' menuitem: Windows Messenger - {FB5F1910-F110-11d2-BB9E-00C04F795683} - C:\Program Files\Messenger\msmsgs.exe
O16 - DPF: {4ED9DDF0-7479-4BBE-9335-5A1EDB1D8A21} (McAfee.com Operating System Class) - http://bin.mcafee.com/molbin/shared/mcinsctl/en-us/4,0,0,76/mcinsctl.cab
O16 - DPF: {BCC0FF27-31D9-4614-A68E-C18E1ADA4389} (DwnldGroupMgr Class) - http://bin.mcafee.com/molbin/shared/mcgdmgr/en-us/1,0,0,16/mcgdmgr.cab
O16 - DPF: {C1FDEE68-98D5-4F42-A4DD-D0BECF5077EB} (EPUImageControl Class) - http://tools.ebayimg.com/eps/wl/activex/eBay_Enhanced_Picture_Control_v1-0-31-0.cab
O18 - Filter: application/x-mfe-ipt - {3EF5086B-5478-4598-A054-786C45D75692} - c:\progra~1\mcafee\msc\mcsniepl.dll
O22 - SharedTaskScheduler: Browseui preloader - {438755C2-A8BA-11D1-B96B-00A0C90312E1} - C:\WINDOWS\System32\browseui.dll
O22 - SharedTaskScheduler: Component Categories cache daemon - {8C7461EF-2B13-11d2-BE35-3078302C2030} - C:\WINDOWS\System32\browseui.dll
O23 - Service: Java Quick Starter (JavaQuickStarterService) - Sun Microsystems, Inc. - C:\Program Files\Java\jre6\bin\jqs.exe
O23 - Service: lxbt_device - Lexmark International, Inc. - C:\WINDOWS\system32\lxbtcoms.exe
O23 - Service: McAfee Personal Firewall Service (McMPFSvc) - McAfee, Inc. - C:\Program Files\Common Files\Mcafee\McSvcHost\McSvHost.exe
O23 - Service: McAfee Services (mcmscsvc) - McAfee, Inc. - C:\Program Files\Common Files\McAfee\McSvcHost\McSvHost.exe
O23 - Service: McAfee VirusScan Announcer (McNaiAnn) - McAfee, Inc. - C:\Program Files\Common Files\McAfee\McSvcHost\McSvHost.exe
O23 - Service: McAfee Network Agent (McNASvc) - McAfee, Inc. - C:\Program Files\Common Files\McAfee\McSvcHost\McSvHost.exe
O23 - Service: McAfee Scanner (McODS) - McAfee, Inc. - C:\Program Files\McAfee\VirusScan\mcods.exe
O23 - Service: McAfee Proxy Service (McProxy) - McAfee, Inc. - C:\Program Files\Common Files\McAfee\McSvcHost\McSvHost.exe
O23 - Service: McAfee McShield (McShield) - McAfee, Inc. - C:\Program Files\Common Files\McAfee\SystemCore\\mcshield.exe
O23 - Service: McAfee Firewall Core Service (mfefire) - McAfee, Inc. - C:\Program Files\Common Files\McAfee\SystemCore\\mfefire.exe
O23 - Service: McAfee Validation Trust Protection Service (mfevtp) - McAfee, Inc. - C:\Program Files\Common Files\McAfee\SystemCore\mfevtps.exe
O23 - Service: McAfee Anti-Spam Service (MSK80Service) - McAfee, Inc. - C:\Program Files\Common Files\Mcafee\McSvcHost\McSvHost.exe

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