Make sure you have the latest video/ audio codecs installed, Microsoft .NET framework 1.1/ 2.0 SP2/ 3.0 SP2 and 3.1 SP1 (but this should automatically install with new games - though not sure if the WoW installer has it bundled) and Microsoft Visual C++ Redistributable.
Other than that, I hope you have a good antivirus solution and a clean PC with a good working copy of Windows.
You didnt give a lot of details regarding what you're running or what exactly "games seem to be slow" means.
Might also want to check and see you are running under Administrator and that the game(s) you are trying to run are cleared or on the exclusion list from the Firewall.
Recently ATI have been having issue with some of their video drivers especially 4 and 5 series cards. It might be best to roll back to a previous such as 11.2 since it seems to be the most stable for 4850 at least since I've noticed issues right away when I moved to 11.3 and also with 11.4 However, they just released 11.5 today as a quick responce to the very much flawed previous version... >.>