Bad PSU?

Hey guys-

I have been having some issues with the old PC and was curious if it might be a PSU problem. I have never had a PSU issue in the past as I always replaced them whenever I did a new build.. in this case I have had the same PSU for more than 3 years...

The other day my computer was working fine when I went to bed but when i woke up in the morning I found it going through a cycle of rebooting.. loading into GRUB.. waiting 30 seconds and defaulting to try and load windows XP. The hard drive would flash and XP starts to load but then the system reboots and the cycle goes on and on.

On the next reboot I tried to boot into Ubuntu and this also failed to boot although instead of automatically rebooting it just hung during the boot up process. At this point I was thinking maybe my motherboard (only a few months old) may have been the issue.

I turned off the PC and went to work, when I came home the system booted into Windows fine and ran ok all night. Just before bed I noticed the infrared light on my mouse which is normally all red flash a few times and then go out and i could not use the mouse at all. I left the room and a few minutes later came in to the system being back in the reboot, try to load, reboot cycle.

Does this sound like it could be the PSU? As mentioned it is a few years old and by far the oldest part of my system with everything else being just a few months old. The room that the computer is in is air conditioned so I don't think it's the summer weather.. although the PC and AC are both on the same circuit and perhaps the AC is not getting along with the PSU?

I saw the advice about MOBO monitor on the sticky so if I can get into windows tonight I plan on looking at that, but would appreciate any other advice.

Thanks

(Sorry for the wall of text)