Alt + ctrl + delete. This brings up the taskmanager, from here you can stop certain programs that are running in the background. But be warned, shutting them all down, or the wrong ones, will results in an unstable system and may cause problems.
For this, the only programs you should close are the ones listed as being used by the account you're legged into. NEVER close programs being run by 'system', 'local machine', ect.
Also, the only program under user that needs to be running in order for you to properly use Windows is 'explorer.exe'. Everything else can be shut down.
It make quality of rendering better.
With it cars have "less" edges etc.
Ok. But I thought GTA4 didn't had a AA function.