When my computer stops working, it doesn't mess around.
Wow. Just had about an hour of explorer not working, and the system freezing up after maybe 40 seconds of being in Windows. Safe Mode fixed it, but I had to force a reboot in the middle of loading to bring up the option of going to Safe Mode. The F8 menu didn't offer it, my CTRL-ALT-Delete menu didn't offer it, the only way in was to deliberately make things worse. Gotta love that.
I reckon it's time to scandisk and defrag now, let Ad-Aware and Norton Antivirus have a look-see. 'cause, maybe something caused this.
(note: XP is still a thousand times more stable than 98 was on my system)