vista install causes mouse keyb failure xp

History: Installed Vista as a "clean install" OVER a functioning installation of XP (not using that drive, but it did have XP on it; it hadn't been formatted). Had only one minor problem in installation; it told me that it would have to disable my SCSI controller, which I thought would be fine, since I don't use a SCSI device.
Current Problem: In Vista, everything works fine except for sound (It does not see my SB Live! plugged in). Doesn't seem to be related to the real problem. When I boot into XP, however (which I have on a different HDD), my USB mouse and keyboard refuse to function! They do not move, click, etc. I can see them in Device Drivers. It tells me that they are working fine. When I unplug and plug back in, I get a beep (as though recognizing the device), but nothing happens. When I open up 'Mouse' or 'Keyboard' in the control panel, they do not show up as being installed. I can use other USB devices (webcam) without a problem; in fact, I can use the 'media keys' on my USB keyboard -- just not the REAL keys! I have fiddled with the BIOS, but as I expected, it made no difference. Help a brother out!