To fully uninstall a software (being a game or a program), follow these simple steps:
1) Locate the software with the Software Manager and click Uninstall.
2) Open Registry Manager, go through each tab and use the Search feature to delete every reference to the software found.
3) Open Registry Finder and use the name of the software as the search word. You should write it with different combinations in order to locate all data. For example, if you are uninstalling Grand Theft Auto San Andreas, you could use for example the following search words: Grand Theft, GTA, Andreas.
4) Run the File Finder with the same search words as the Registry Finder. Delete all files relating to the software.
5) Run the Registry Cleaner with the Normal mode.
Word of caution: When using the Registry Finder (and File Finder), avoid using search words that might show items not relating to the software. For example, if you are removing a software called "Font Manager" don't search with search words "font" nor "manager" because both of them are too generic and will most likely show items not relating to the software.
Always let the program create backups of items you remove and don't remove anything you are not sure is safe to remove.