XP random lock on Compaq Notebook

I have noticed that windows xp pro has been locking up on me at seemingly random times. For example, i was on this thing for 20 min, and it locked up. I rebooted, and was on for two hours before it locked. I have been running off the ac adapted (the battery is bad, and waiting for another one). I dont have a lot of stuff installed (just windows, compaq drivers for xp, and sp1a.) this is beginning to irritate me,..... any ideas?

 

Any clues in your event logs?

Locked Up as in nothing moves or as in the mouse moves but does nothing when clicked?