I'm not sure if I understand your question, but if you mean that you have no mpu-401 driver anymore in your windows settings, you can re-install the driver from your windows cd.
Which version of windows are you using?

fred
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Keyboards/Sound Units: Kurzweil 2600S, Roland VR-760, Acces Virus C, Roland G-800, Akai AX60, Minimoog, Machine Drum, Roland R8-M, mediastation x-76