By disable default security set, I assume you mean disable the User Account Control? Did you install the game before or after you disabled UAC? If it is before, you may need to reinstall the game, as disabling UAC will sometimes mess with the settings for already installed games.
You can also try to copy the file out of the Program Files folder to your desktop, uncheck the Read-Only flag, and copy the file back. I doubt it is a virus, but you can always run a scan to be safe.
- Star Admiral