Old Demesion C521 Service Tag wrong

Probably doesn't matter...

There is a Service Tag Utility with hidden switches.  You need a WIN9X Boot floppy to run it. Or make a bootable floppy

http://www.dell.com/support/home/us/en/19/Drivers/DriversDetails?driverId=85WWG

https://downloads.dell.com/utility/ASSET_A209.COM

HERE'S HOW YOU DO THIS.
Make a bootable dos thumbdrive. 
Download asset_a209 file located on

http://downloads.dell.com/utility/

asset_a209.com and put it on your bootable thumb drive.
Boot the pc with this drive.  It'll come to a command prompt.
asset /s /d
y for yes.
This will delete the service tag.

Want to change the service tag?Do this:

Startup in MS-DOS and run your asset.com file from there.

Usage: asset.com /s XXXXXXX

(where XXXXXXX is the new service tag name)

You can use the Asset tag utility to do this.

There are hidden switches to asset.com to change the service tag.

ASSET.com version 2.09 from the resource CD would be the likely candidate.

ASSET 209   Asset Version 2.09
Release Title: System Utilities:
Dell Asset Utility, Utility, English, Multi System, 2.09

http://www.dell.com/support/home/us/en/19/Drivers/DriversDetails?driverId=85WWG

Release Date: 09/27/2005
Description: Asset Utility

http://downloads.us.dell.com/utility/R109071.exe

Utility : Dell Asset Utility
Version 2.09
For DC051, DXP051, DXC051, DM051, DE051, DXP061, DXC061, DM061, DV051;
Dimension : 8300 / 8300N, 2400, 4700, XPS Gen 3, 8400, 4700C, 5000, 3000, 9100,
XPS (Gen 5), XPS Gen 4, 5100C, 5100;
OptiPlex : SX280, GX630, GX530, GX620, GX520, 210L, 170L, GX280, 160L,
SX270 / SX270N, GX60, GX270 / GX270N;
Precision : 360 / 360N, 670, 470, 370, 380, WS390, 690, 490
On Not Applicable
 

boot from dell resource CD
choose "Q" to quit the diagnostics and go to command prompt
navigate to "asset209.exe" tools
cd csd\tools\bios
run "asset209.com"

Usage:
ASSET new_asset_tag Specify new asset tag
ASSET /d Delete asset tag
ASSET /o new_owner_tag Specify new owner tag (if applicable)
ASSET /o /d Delete owner tag (if applicable)
ASSET /? This help screen

Hidden Options

ASSET /s new_service_tag Specify new service tag
ASSET /s /d Delete service tag
ASSET /b Specify new system board tag
system board tag must be "ssssssssssssssssssss-vvv-", where
s..s is the 20 digit system board Part number string,
vvv is the 3 digit system board rev,
- is a don't care character, but must be present
ASSET /b /d Delete system board tag
ASSET /f [other_options] Force Affirmative response
ASSET /x [other options] eXtended display - displays all tags