Vista install causes USB mouse/keyb failure in 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!