Since this is a USB keyboard, try a PS/2 keyboard, assuming your computer has a PS/2 plug, of course.
If not- or if the PS/2 keyboard doesn't work either, try this-
Open Device Manager (right click My computer, "Properties", Hardware tab, Device manager button)
In device manager, open the "keyboards" node (icon).
Right-Click and select "uninstall" (your keyboard might have a different name then mine, of course.) Also, if there is more then one item within the "keyboards" section uninstall all of them.
Once you've done this, reboot the computer. With any luck, windows will detect the keyboard and reinstall the proper drivers and so forth. If not, try to jog it's memory by unplugging the keyboard and wait until the PC makes a sound indicating removal. Then plug it back in. With luck, the computer will recognize the keyboard and you'll be good to go.
if none of the above work, it could be a USB issue. To try to fix that try uninstalling all "USB root hub" and "USB controller" entries in the "Universal Serial Bus Controllers" section in Device manager, and rebooting.