You don't mention what type of keyboard it is or how it connects to your computer but have you tried disconnecting it and reconnecting it and/or possibly updating whatever software is appropriate?
It's the standard keyboard which came with my HP computer and has always functioned well. But it is clearly not the keyboard itself. I've connected and reconnected and attached the keyboard successfully on another computer. Conversely, I've taken an identical keyboard, attached it to the affected computer, and it does not function. Most importantly, the keyboard is no longer an option as a device. It's a software issue. Something wiped it out. And, again, it occurred immediately after installing the McAffee update.
It could be that VirusScan has quarantined the driver and.or software, why is anyone's guess. Double-click the taskbar icon to open SecurityCenter
Click Adcanced Menu (bottom)
Click Restore (left)
Click Files to see if there's a clue there.
Try restoring it.
Is this a USB keyboard or PS2?
Does the mouse still work?
PS2. Mouse still works.
Which Operating System are you using?
98, XP, Vista etc?