Run System File Checker (SFC)
System File Checker (SFC) is a Windows built-in tool that checks that the Windows system files are in the same condition as they were when Windows was installed, and not damaged, corrupted or changed.
If a corrupted, changed or damaged system file is causing WMP to stop working, SFC will find that file and let you restore the original one from your Windows 7 install disc.
To run SFC, follow these steps:
1. Go to Start Menu -> All Programs -> Accessorries, right-click on Command Prompt, and select "Run as Administrator"
2. If prompted for an Administrative password, enter it and then click OK
3. When the Command Prompt window opens, type "sfc /scannow" and press Enter
SFC will start scanning the system files. Note that this process will take some time, so be patient.
4. Once the scan is finished, test WMP and see whether the problem that you’re experiencing is resolved.