When in Windows XP, or sometimes during a reboot trying to get into BIOS(unsuccessfully) , the keyboard will not work or stops working. PS2 mouse keeps working though. Same thing happens with another PS2 keyboard. Turning off computer ( through windows shutdown) does not help, nor does rebooting. However, shutting down the computer with no power for about 45 seconds ( too short of a time with the power disconnected does not work) ALWAYS brings back keyboard function. It appears that something in memory needs to be refreshed. I have a Gigabyte GA EP35p-DS3R MB, 4 GB Corsair mamory, and an Intel E8400.
The obligatory "If it is a wireless keyboard then check batteries and wireless receiver".
Another poster suggested the following,
"Another thing to try is to delete the KBoard driver in Hidden device (in
Hardware manager/ Show hidden devices) since XP keeps a copy of any driver
it needs to boot. Might be a corrupted one so it will reinstall from CD or
CAB."