As said - try another keyboard.

Also try going to START > CONTROL PANEL > KEYBOARD, click on HARDWARE tab, then the PROPERTIES BUTTON and finally the DRIVER tab. Try looking for a better driver using the driver button, or rolling back the driver to a previous one by using the Roll Back Driver button....

Failing all that, go to START > ACCESSORIES > SYSTEM > SYSTEM RESTORE and restore the computer to a previous date when you remember the keyboard working fine....

The last option is a tad extreme tho.... I;d just get a new keyboard.....