BleepingComputer.com: Strange crash after startup

Hi all,

I have a computer with Vista Ultimate installed on it.

Here's the problem:
After start up, the desktop loads. This window is already visible:

(note, screenshot taken in safe mode where it also appears)

The cursor is a loading cursor, when this return to a normal cursor the computer crashes. It's not a BSOD, it's a load of lines across the screen just before the computer then restarts itself. I am able to boot into safe mode just fine. Below is the events leading up to this behaviour.

Here's the story:
Yesterday I had a problem with a virus that AVG flagged as Agent_R.AOB. AVG wouldn't remove it, it wouldn't find the related file. I messed around with that for ages, finally Avast managed to remove it in safe mode, or at least it removed something. I also ran Malwarebytes which fixed found and fixed some other infections. After that the computer was rebooted and worked fine for several hours. This morning when it was booted it crashed as described above, and has continued to do so. Boo.

Tried so far:

• Booting into safe mode - this works fine. I'm in safe mode now.
  
• Removing the BIOS battery, sometimes static charge (or something) builds up an the comp refuses to boot until I've taken the BIOS battery out and fondled it. That usually fixes the issue. Not only did this not fix the problem, it caused the computer to fail to boot even the bios until I also removed the graphics card and replaced it in it's socket.
  
• Updating and running AVG
  
• Updating and running Avast
  
• Updating and running Malwarebytes
  
• Running sfc /scannow from a command prompt I can't do another one of these because when I do it tells me the computer is waiting for a reboot before I can. Have rebooted dozens of times, though not without crashing ;-) Have rebooted into safe mode though.
  
• Diagnosing memory issues with the Vista install disk
  
• Repairing startup issues with the Vista install disk
  
• Starting normally with startup items disabled and all non-windows items disabled.

Note I can't repair the installation from the install disk for whatever reason. That is, I can't do a fresh install over my current one.

Anyone got a diagnosis? I feel like I'm running out of options!