http://www.tech-recipes.com/rx/7168/how-to-overwrite-system-files-in-windows-7/
1. Click the Orb (Start menu), type in cmd, right-click on the cmd.exe and choose Run as administrator.
2. Now, you must take ownership of the file by typing the following command:
takeown /f C:\Windows\System32\wmpeffects.dll and hit Enter (replace C:\Windows\System32\wmpeffects.dll with your file)
If succeed, you will see a message like this:
3. After that, type in the following command. This will give you permission to access the file:
cacls C:\Windows\System32\wmpeffects.dll /G jessica:F (replace Jessica with your username) and hit Enter.
It will ask you whether you sure to do that? Type in Y and hit Enter again.
4. Now, you can easily overwrite system files without any problems.