Windows 7, default admin account, UAC enabled, game installed to program files directory?
If that is the case, then the problem is likely related to Integrity Level.
Starting from Vista, every securable object has an IL. The default is medium but some important stuff, like program files directories and windows directory, is set to high. Every process also has an IL. The default for standard user and admin under UAC is medium. Only process with equal or higher IL to an object can modify it. That's why you need to elevate an installer to install a program.
So if you want to modify high IL files, you need to elevate the program you use to make those changes.