Mouse & keyboard unresponsive

Usually occurs when kids have iPod hooked up, but has happened randomly
also. USB on front of machine being used, not keyboard's.

Display is not black(like when power saver kicks in) but kind of a light
gray which tells me above has occurred.

Mouse and keyboard have no reaction when used, yet I can go to one of
the other machines on the network, access files, etc. Doesn't seem like
it has locked up yet will take no input or respond to it's own keyboard
or mouse.

Only way I've found to get it back is to power down and reboot.

Ideas appreciated.

Thank you.

Mark