First of all, I would not downgrade WMP 12 to 11, I do not see a good reason why.
http://www.microsoft.com/windows/windowsmedia/download/AllDownloads.aspx 1. Type services.msc on windows 7 search bar
2. Stop Windows Media Player Network
Sharing Service.
3. Take Ownership of following files
All files in “Program Files\Windows Media Player\” folder
All files starting with “wmp” in System32 folder
wmploc.dll.mui in “Sytem32\en-us\” folder
4. Now Run Windows Media Player 11 Installer
5. Now type Regedit on Start Menu Search bar
6. Delete following Registry key
HKEY_LOCAL_MACHINE\Software\Microsoft\MediaPlayer\ Setup\Installed Versions“
(Don’t forget to take Backup of Registry before modifying/editing/deleting any registry key)
7. Reboot your Windows 7
Download Windows Media Player 11 for Windows 7 (Replace WMP 12) Windows Media Player 11 for Windows 7 - WinMatrix ~~~~~~~~~~~~~~
Again, I personally don't think it a good idea, it's just what some people prefer.