This has happened to me a number of times now. All of a sudden, plug and play hardware stops working and when you go into the device manager it all appears blank. Microsoft KB311504 suggests that this is caused by the Plug and Play service being stopped but this was not my problem.
My problem was that the HKLM\System\CurrentControlSet\Enum registry key was lacking permissions. Use REGEDT32 (Windows 2000) or REGEDIT (Windows XP) to add the System account with full control and Everyone with read to make this work again.