There are two possibilities here:
1) It's a problem with Windows
2) It's a problem with your PCs USB ports
The easiest way to see if your keyboard works fine is to try and access safe mode by pressing 'F8' just before the black 'loading windows' screen appears. You could test the numlock and capslock keys too while the PC is loading.
To further test the USB ports in Windows, you could try plugging in an MP3 player or a flash drive or whatever, just something to test. You might also try unplugging the mouse and keyboard in Windows and moving them to different USB ports.
If your keyboard works fine prior to Windows loading then you know it's a software issue. If it's not working at all, then you know it's a hardware problem. In the case of a problem with Windows, you can boot from your Windows DVD and attempt to fix the problem that way.
If the USB ports just aren't working at all (even before Windows loads) then you'd need to call Dell and deal with them directly since they built the machine.
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