Had a similar thing a few days ago.
I installed the trial version of Panda AntiVirus. This prompted me to remove other (Norton) AntiVirus. As Norton has expired anyway, I removed it and installed Panda.
After the required reboot I logged onto the web (via AOL)
Boum, PC shut down during the password sequence.
It kept doing that until I remove the Panda antivirus programme again and re-installed the (expired) Norton programme which I had backup on a CD.
After re-boot systems worked OK again.
I am using Winddows XP (+SP2) and guess it has something to do with XP security settings which knew I has Norton installed but when it did not discover it after my deletion it shut down as a precaution?
Message shown after re-launch with Norton uninstalled was:"Your PC has just recovered from a serious error, please report it to microsoft".
BTW, I have run Norton/AVG/SPYBOT/Ad-Aware/MacAFEE and ZoneAlarm together for many months without any probs.
Maybe someone can confirm/deny the Window XP security action in the above scenario.
Could be Sky_Captain's problem as well?