XP MCE hangs after entering password (sometimes)

Hi all,
I have an odd problem with one of my computers. It's a relatively newly upgraded machine, AMD PhenomII X2 555 BE, Gigabyte mobo, running WinXP Media Center Edition, new 1TB WD Caviar Black drive. It's working pretty well most of the time now, but... about 10% of the time, it stops responding when you put in your password from the Welcome screen. The hard drive light blinks a bit, like usual, then nothing more happens. It does not get as far as the "loading personal settings" before it stops. This happens with more than one user account, both limited and administrator privileges. Since it's not every time, the troubleshooting is not easy. I have cleaned up the leftover video drivers (which were causing beeps during the "Windows XP" screen before, but that is now gone), done spyware/malware/etc. scans with MBAM, cleaned registry errors with CCleaner, etc. I don't think there is bad stuff on there, but Driver Verifier found a problem with one driver that I don't recognize (PsSdk30.drv).
I'm trying to fix it before breaking down and doing a fresh install of all of the software, but I can't do an in-place restore since the OS disks are SP2B and the computer is now SP3.

Does anyone have any clues where I can look next?