Have you tried

1. System File Checker

To run System File Checker, do the following:
a. Click Start, click All Programs, click Accessories, and then click Command Prompt.
b. In the Command Prompt window, type Sfc /scannow.
This causes the System File Checker tool to verify all protected file versions and add all absent protected files to the Dllcache folder.

more info

OR

2. System Restore

To use the System Restore Wizard, make sure you're logged on as an administrator, and then follow these steps:
a. Click Start, point to All Programs, point to Accessories, point to System Tools, and then click System Restore.
b. On the Welcome screen, click Restore my computer to an earlier time, and then click Next.
c. On the Select a Restore Point page, select the date from the calendar that shows the point you'd like to restore to, and then click Next.
d. On the Confirm Restore Point Selection page, verify that the correct restore point is chosen, and then close any open programs.
e. Click Next if you are ready to proceed or click Back to change the restore poin.
f. The computer will shut down automatically and reboot. On reboot, you'll see the Restoration Complete page, and then click OK.

more info

OR

3. Reinstalling Windows Media Player 9/10