PSU going out?

I left my computer on during the night and when i came back the ASUS probe was up and the +3.3V had gone over the threshold at some point during the night. It was the first time it had happened. I went to play Diablo 2 (not exactly taxing to a system) and within 10 minutes the same thing happened 3 times. I have had my system for less than a year, the psu came with the case, it's an antec 400w performance Model SL400. Is this anything to be concerned about? Just while writting this post it occured and the only thing in the background is Kazaa, norton, and the asus probe.
ASUS A7V333, 2200+ XP, 512 pc2100 ram, geforce4 Ti4200

The threshold is set at 12%(originally 10% but i bumped it up so it would beep less often), my system isnt acting strangely aside from that, although i did just recently get cable internet so i now have a usb receiver(the router/modem are on a diffrent computer). That is the only thing that has changed, i had never had that occur before. Can you recommend something that is more accurate?