System freeze - battery or MB?

I'm running xp-sp2 on an Asus P4P800 Deluxe, 4 x multipartitioned IDE disks, 2x dvd readers, no RAID.

My system froze completely while a program was running. No BSOD, just the normal XP background with all the icons. When I rebooted, I couldn't even reach the BIOS setup stage. Tried sevaral times to no avail.

I removed the desktop side panel and let the pc rest for about 3 hours, just in case there was a cooling problem.

When I rebooted, everything worked fine except that the System date and time had moved back to 2002, which is the year when the MB was produced (I actually bought it 5 years ago, no problem at all ever). Furthermore, the power
supply fan appears to be running fine too. With the panel removed, the pc has been running smoothly for the èast rwo hours.

This is probably a symptom that something is going to let go. Could you give me a hint as to what may be defective ?