This is a total Gloat Moment: A client called this week, saying her [shudder] Windows XP machine was restarting itself moments after it had been switched on. I suggested a Safe Boot, but then she said the behavior had changed; she could start up, but before sue could enter her password, it told her the password was incorrect. I took a shot. “Could you grab a different keyboard from another computer in the office?”
“That’s not it.” “Do me a favor and try it.” Victory! Reminded me of an early gig I had, when a Mac was freezing up. The keyboard’s wire was totally exposed and shorting out. And one time, a MacBook Pro I owned wouldn’t stay asleep; I would find it in my bag completely hot and battery-depleted. Turned out an ExpressCard card reader I had was screwing it up. Hardware is a fragile thing. Don’t go reinstalling ’til you’ve checked out the peripherals!
jjmarcus tech solutions 
I enjoyed you comments about checking the peripherals first. I used to work for Radio Shack. A woman came in the store with one of RS’s famous radio/tape recorders that required batteries or an adapter. She said the tape didn’t work, but the radio did. I suggested we check the batteries, but she insisted that the radio worked, so it couldn’t be the batteries. After adding new batteries, everything worked fine. There just wasn’t enough juice to power the tape mechanics. So my philosophy has always been, “check the batteries, first!”.