Skype does not install itself using multiple registry keys or files. The setup package installs only one file (Skype.exe) into Program Files\Skype. If you delete that file and the shortcut associated the program will be removed from your computer. It's the easiest way and it doesn't have a bad impact on your system's stability or you could use its own uninstaller as well. If you feel confident that there are more files that need to be removed, you can try using CCleaner to clean leftovers.