Well a couple of obvious things here 1 would be to change the BIOS Battery so you know that the one in there isn't nearly flat this can cause all kinds of problems when this occurs.
If you have recently fitted a new HDD it may not be compatible with the BIOS and you might need to flash the BIOS to a newer version. To do this you either need the M'Boards Makers Name & Model number or the System Makers Name Model Number & Serial Number then visit their Web Site and download the newest BIOS Upgrade available and Flash the BIOS. You'll also need to read the direction on how to do this as different BIOS makers have different ways of performing this operation.
It's also possible that some Drive Makers Drive Utility was used because the drive is bigger than what the computers BIOS can see so the BIOS has effectively been tricked into not seeing a HDD because the Drive Makers Mounting Utility is in operation. If this is the case it's quite possible that a new MS Patch has broken the links between the HDD makers Utility and Windows preventing your Keyboard from working.
I would shut the system down unplug the mains for at least 3 minutes and then try again that may fix your problem. Failing that you'll need to remove the Keyboard from the Device Manager and allow Windows to find it on the next reboot and install the necessary drivers which provided that the keyboard is still working will allow the system to work properly again.
Have you tried this computer with a [b]Known Good Keyboard?[/b]