Try checking your Bios settings...maybe you should also get XP to synchronize when you are on the net, and update the atomic clock automatically.
I would have thought that if it was setting it back randomly it was the bios. Does it set it back during work, or when you boot up it doesn't seem to be correct then. If it's the latter definately check the Battery and/or replace (Bios battery that is)
Cheers...hope this helps