Try this, click the Windows Orb (Start) > All Programs > Administrative Tools > Services > General Tab. Scroll down to, and double-click the Plug and Play service and make sure it is set to Automatic and that it is running, click OK, etc. and restart the computer.
If it’s not a Service and it’s not an infection, the only other thing I can suggest is to run the System File Checker (SFC).
Run it from an elevated Command Prompt, i.e. click the Windows Orb (Start) > All Programs > Accessories and right-click Command Prompt, then ‘Run as Administrator’. Type sfc /scannow (note the space) and press Enter. This may take some time and, depending upon your installation of Vista, you may have to put the Vista DVD in the drive.