I am having the same problem... except this has happened with 2 keyboards on 2 computers.
Last year, I was using an old Compaq Presario 7000 that we had purchased in 1999. It had been re-fitted with Windows XP in 2004 and it was the computer I used up until Christmas of last year. Starnge things started to happen with it's keyboard though.
The computer itself was littered with spyware, and since I had never been able to flush it out with the free spyware programs it lingered until the very day it was replaced. But on to the matter at hand, the keyboard stopped working suddenly after startup. It worked fine in safe mode and worked on startup.. but once it got to the welcome screen it stopped functioning. Re-plugging it in didn't work as it would make the hardware fail sound, and not accept it. My old keyboard was a USB, and figuring the keyboard was faulty I threw it out and I was able to use the one that came with my Xmas Computer. (Mom let me get it. :D)
For a small time, it worked. But not long afterwards it started to fail too. And like the poster above, I did everything i could think to make it work, and finally I was able to do it. But it was an odd way to do it. I made a guest account and re-plugged in the keyboard and was able to get it working on the guest account.
Time passes and Christmas comes and I get to use my new computer! Using the keyboard that had come with it (The same I had been using with my old computer for a time) it worked perfectly until it too fell victim to this strange error. Can keyboards carry over malware? I sent some files over from my old computer, and it likely has something to do with it but spyware scans halt nothing. I am totally at loss and I really don't have the money to buy a new keyboard, let alone knowing that it too could succumb to this failure. Can someone please give insight?