Dell Dimension C251 USB not recognized

All right gurus and newcomers, I've got a tricky one for you all to ponder over. I have been asked to work on this system, a Dell Dimension C251. A few days ago I was asked to clean out all of the malware/spyware that had infected this system. In the owner's/user's home, the keyboard and mouse worked. The only problem with the keyboard was that the the Windows/Start button wouldn't bring up the menu in Windows.

I have taken the computer, but left the mouse/keyboard at the owner's home. I connected my own keyboard and mouse (via USB - no PS/2 ports on this system), but when Windows XP Media Center gets to the login screen, there is no response from mouse or keyboard. Both devices are powered up, but it is as if Windows XP M C failed to load USB. I will be taking the now clean system back to the owner, but should her mouse and keyboard not work, I'd like some suggestions.

Here are the problems to work around:

1. CD/DVD drive is busted, I can here rattling from within the drive
2. No PS/2 ports to connect mouse/keyboard
3. The system works fine up until Windows XP M C login screen - mouse and keyboard stop working at this point, but keyboard works fine in BIOS
4. Trying not to buy new devices
5. Don't want to reinstall Windows, yet

I have never heard of a virus that disables USB drivers, but if someone has info that they can provide or link, I will look into it.

If there are any questions, just ask.

Thank you, all.