The date on my Windows 7 x64 PC has twice been set to the year 2028, which causes all sorts of stuff to stop working (website certs, TV guide listings, virus definition updates, etc). Any idea what would cause this?
There are several solutions, that one of them will solve your problem for sure.
To replace the CMOS battery, you'll have to follow these steps (this was copied from some previous answer of mine, that I posted here in Super User):
That's it. Just plug in the battery, and see if it solved your problem.
Some broken BIOS settings may cause it, but it's more likely it's a hidden software problem.
I would recommend to check the synchronize checkbox at the time settings page in WinXP, that will fix your clock by synchronizing with a NTP (Network Time) Server.