Start here:
http://support.microsoft.com/default.aspx/kb/314060
If that doesn't work, try this from MS also:
"Method 2: Replace the existing driver
1. Click Start, and then click Control Panel.
2. If Control Panel is in Category view, click Performance and Maintenance, and then click System.
If Control Panel is in Classic view, double-click System.
3. On the Hardware tab, click Device Manager.
Note In Windows Vista Beta 2, click Device Manager in the left pane. If a dialog box appears that describes permissions, click Continue.
4. Click the drive with the question mark next to it.
5. On the Action menu, click Uninstall. When you are prompted to confirm the removal, click OK.
6. Repeat steps 4 through 5 for any other drives with question marks.
7. On the Action menu, click Scan for hardware changes."