SecuROM
Current versions (i.e. v7) has 2 modes of running.
If you run as a non-admin, it’ll install a service called UAService7.exe, which works in ring 3 mode (not so bad, and works around one of my pet peeves, i.e. requiring administrator mode to install, or worse, run the game.
If, however, you haven’t wisened up to the benefits of not running as an admin, it’ll install a driver in ring 0, which basically gives it full access to whereever it might want.
For more information: http://en.wikipedia.org/wiki/Securom