I highly doubt the Windows Index is the cause of this problem. Sounds like something just got messed up or overwritten.
If you need to see a list of the Windows Media Player components and codecs installed on your computer, do the following:
On the Help menu, click About Windows Media Player.
If the Help menu is not visible, click Organize, point to Layout, and then select Show menu bar.
On the About Windows Media Player dialog box, click Technical Support Information.
Your web browser will open a page that includes information about the related binary files, codecs, filters, plug-ins, and services installed on your computer. A technical support person might be able to use this detailed information to help you troubleshoot problems on your computer.
Look at the Codecs section and post here if you want. You can try to install a codec package (that I use myself and have been for years) K-Lite Codec
I read that there are some other methods to possibly restore default codecs, but I'm not exactly sure they will work.
or you could try the Windows Vista Media Center update process (which apparently also works in Windows 7):
1. Close Windows Media Center
2. Click Start and type cmd in the Search programs and files box
3. Right-click cmd.exe from the Programs list and clickRun as administrator
4. Type the following command at the prompt and press Enter
start /wait %windir%\ehome\mcupdate.exe –uf
5. Wait for the command to finish, then close the command prompt window, restart Windows then restart Media Center and check for the issue
For the above my computer didn't have 'ehome' directory, but a mcupdate.exe was located within the winsxs folder: