Bad port where you plugged it in? Had you tried moving it to another USB port? Or are you using a PS/2 to USB adapter?
i've tried everything..from visual inspection(looking for bad capacitors and any damage) to flashing bios. old PS/2 keybard included (w\o any adapter).at first problem looks like related to network on board card. i did install a spare one- it did solve the problem. but only for couple days. if you goggle enough for this particular board problems-you find that a lot of folks have same thing. and no one was able to figure this one out. at least i was able to narrow down the defective part.