in order of likelihood:
1. Creative Sound Card? SB Live set the standard for crappy cards that will screw up everything on a motherboard. They may have cleaned up their act since then, but pure reputation makes them #1 suspect in any computer problems: they can bring your whole system down. So, do you have one installed now? Has a Creative sound card come within 100 feet of your desktop in the last year? Hell, I replaced a whole MB/Memory/CPU once, and it turned out to be a Creative sound card.
2. Power supply. Usually, you'll see PS trips (or BSODs) ... but a bad PS can cause any number of issues.
3. MB. My current PC burned off a couple of USB ports when I first plugged it into the wall. Have you smelled anything vaguely burning recently? Open the PC and stick your nose on various parts of the MB. Does any region smell?