Quote Originally Posted by MrTroy View Post
It could very well be the motherboard, especially if the mouse is not getting power from the USB or PS2 port. I assume this is a optical mouse with a red glowing light?
I had thought of this just a couple minutes ago, so I opened my case and booted the computer, and sure enough no mobo activity. Everything that wasn't linked to the mobo was powering up no problem. I disconnected and reconnected every cable, with no luck. So while the computer was on, I messed with the cables and jiggled the bulk of them up near the PSU, and bam, power. So I guess I've got a bad, loose cable in my PSU. I'll be replacing this ASAP.

Problem solved!