Tip 2925 links to a Microsoft article on troubleshooting similar problems.
On my laptop, I have been unable to Eject the PC for some time and I just discovered the culprit.
When you install Windows Media Player 7, the default install settings include installing 2 drivers for the Adaptec compact disc recordable (CD-R) plug-in. These drivers prevent my laptop from ejecting, and may cause Standby or Hibernate problems on other computers.
To fix the problem:
1. Uninstall Windows Media Player 7.
2. Check the Allow removal of shared components box and press Yes and Next.
3. Check the Adaptec... box in the Components area and proceed with the uninstall.
4. Restart your computer.
5. Re-install Windows Media Player 7, but uncheck the Adaptec... box, when the list of plug-ins is displayed.
You can download Windows Media Player 7 from http://www.microsoft.com/windows/windowsmedia/en/download/default.asp.