OK........here's one I'm baffled by. My clock (bottom right hand corner of the screen, on the tray) seems to show different times when I boot up. Any ideas? It doesn't happen all the time.

I use Windows XP Pro with a P4 @ 2.4 ghz.

This seem to be a problem with earlier version of XP. After a while I am pretty sure they fixed this in the windows updates. XP now has an tab at the top when you go into your clock settings to set up the auto update for the time.

Either that or your bios configuration is being changed when you boot, though this is a remote possibility. However, I mention it as a very cheap and extremely quick thing to check, as if this is the case it is most likely the result of a dead battery which would require replacing (the little button shaped metallic battery on your mainboard - be careful while lifting it out, and inserting a new one)