I found that Vista has issues at times with KVM, try and boot the Vista system first log on then switch to XP system and log on then switch back and forth and look for error's. If this does not work be sure to check your windows updates on your Vista machine and make sure there up to date. Davidanders brought up a good point some USB do not put out power you can also try using a different USB port. hope this solves the problem.
Edited by Tim Ace, 18 June 2010 - 12:23 PM.