Unbootable condition of PC

Helo everyone!

I had Dell Optiplex 390 with windows 7 preinstalled, few days back it got 0x0000007B BSOD error. I have tried many ways but can't get out of it. Even my system was not booting from USB, DVD etc.

So I added my Laptop HDD to it with windows 8 installed. Now I can boot my computer but this windows 8 version is on 32 bit and I require 64 bit for my previously running softwares. Now when I try to update from 32 to 64 bit. my PC can't boot again from USB, DVD etc but only boot to the new hard drive with windows 8 32 bit installed. Please help I'm kind of stuck at this version of windows, tried update from many ways but no result.

Kindly help!

Thanks.