Blue screen after logon

Hi everyone,
I am new to this arena so please bear with me. I work for a company whose network uses some Dell optiplex 745's running windows xp service pack 3. Recently one of these developed the BSOD. It comes up after logging on with username and password. It will not let me ctrl+alt+delete. the only thing I can do is restart which is when the problem repeats.
I have thouroughly checked out the bios settings, performed hard drive and memory diagnostics and I have put in a new HD from western digital: SATA 250 GB, 2500AAKX.
I have also reimaged the computer which works fine. Basically I have tried everything I can think of, even checking out different monitors and hard drives.
I am able to boot in safe mode and run an Anti-virus scan which came back clean.
I was able to do a manual scan in safe mode using symantec endpoint protection and the only thing that came up were some tracking cookies.
I am of the mind that there is either still a problem with a virus or there maybe issues with the integrated graphics card.
No other computers on the network have this issue, it is localized to this one desktop.
After all of this the issue still persists.
I feel like I am missing something and if I do not figure this out soon, what will be missing is my hair! I am about ready to pull it out by the handful.

If anyone has any Ideas I would sure like to hear them! Thanks alot in advance!
 
Thanks!

Thanks TMagic650. since my initial post I have found out that it was hardware compatibility issue. I have since switched the offending machine out with Dell optiplex 755 using the new hard drive. the 745 was having issues with overheating chipset and fan failures. I also found out that the replacement hard drive that was ordered has some compatiblity issues with the 745's but works fine with the 755's. weird, huh? thanks for your quick response.
 
Back