I was using Vista 64 bit when this first happened (new computer btw):  I would boot up and the keyboard wouldn't work; sometimes it would be the mouse (sometimes both - honestly, it's mostly the mouse (it seems)).  It's also done it while I'm doing stuff (mostly during bootup tho).  The only way to fix it (thus far) is reboot or unplug the mouse and replug it back in.  Rebooting fixes it btw; without me having to unplug/plug in the USB stuff.
 
Now I thought it was Vista - so when I installed Win7 (clean install - FDisked the drive) a few months ago, I found out it's still doing it.  That leads me to believe it's something on the motherboard.
 
I'm going to try flashing the bios with P10 tonight and see if that helps.  I doubt it will as I've already flashed it with 08 and 09 (it came with 07)
 
Anyway, any suggestions?  Maybe it's a BIOS setting?  Currently using MS 4000 ergonomic keyboard - and Sidewinder mouse (first generation)