How to keep Windows running at its best?

I built a Pc, it ran great for a while with WIndows 7. I upgraded to Windows 8 and I use AVG Internet Security for Anit-Virus and I don't download anything untrusted. I play alot of games like Battlefield, Cod, MInecraft Etc. After about a month it is starting to slow down, alot and sometimes it freezes. I do Virus scans every 3 or 4 days and AVG never finds anything. What more is there to keeping WIndows running fast, My computer is always organized and there isn't tons of junk.

One of the oldest commands in Windows surely still works in 8 (I've never used 8 and likely never will) - Ctl/Alt/Delete all at the same time. Your task manager box will pop up. Look at the processes that are running and then look at the services that are running and some of the processes are running because the service is running. You need to know what you are doing to a degree to change this, but you can turn off services in control panel. Running unnecessary services is one of the biggest reasons computers run slowly. If you turn off most services and something goes haywire you can just turn them back on, but be careful.