My WinXP x64 would freeze after typing in username/password - the color bar stops moving. Or it'd freeze even after just clicking "Shut Down" - the button would stuck.
One thing seemed to help is unplug KVM after forcing it to shut down, but it may just be a coincidence. Unplugging KVM before starting doesn't seem to help.
Today it got worse - XP actually reset it self a few seconds into the logo stage. After a few safe mode boot, again unplugging KVM seems to be helping. The ntbtlog.txt doesn't contain anything interesting.
System log showed consistently driver blocking for pfc.sys and GEARaspiWDM.sys under SysWOW64. They seem to be from Intervideo and iTunes.
I disabled Application Experience Lookup service, just because all failed start (freezing) seemed to stop right before the AEL event, compared to the successful ones. Also disabled WWW, Image Acquisition, Proxy Auto-discovery, and Error-reporting services, just for the sake of it.
The only major changes I remember happening before this issue is installing PerfectDisk and x64 SP2.
PS. The computer so far stayed healthy. It may have just been the AEL. Since MS doesn't really say how AEL works (or even what it does), it's probably no big loss without it.
PS. (07-31-2007) Problem came back. Now I just hibernate the machine and it didn't have the problem when waking up. However, for some reason I have to shut down the PSU; otherwise the machine will restart 3 seconds after hibernation.