Copied from the Microsoft Knowledge Base article,
No Items Appear in the Device Manager List When You Open It:
1. Click Start, click Run, type services.msc, and then click OK.
2. Double-click Plug and Play.
If you receive a Configuration Manager message, click OK.
3. In the Startup Type list, click Automatic, and then click OK.
4. Close Services.
5. Restart the computer.