It looks like the new virus definitions pushed out by Mcafee today are detecting a false positive of "W32/Wecorl.a" in svchost.exe (
http://community.mcafee.com/message/125600#125600) . When it kills this process Windows forces a reboot after one minute, displaying a countdown timer. Here's how to work around this issue:
1. Log in to Windows before the reboot is triggered.
2. Once you get the reboot countdown timer, pull up a command prompt and enter "shutdown /a" to abort the reboot
3. Open "start->program files->mcafee->virus scan console". From the "tools" menu, select "Rollback DAT" to downgrade to the previous virus definitions.