I'm using AVG free; hard to beat the price/performance.
If you dont have an internet connection, you dont need an antivirus. If you dont care about losing everything on the HDD, reformatting and re-installing occasionally then you dont need an antivirus. If you play any online games, you better have one as a basic precaution against a keylogger stealing your account passwords. If you are running a new x3 or x4 CPU, it wont slow your gaming down noticeably, even if it starts a scan in the background.
Spyware is more often obtained by browsing or downloading. If you do internet browsing to look up game info then its probably a good idea. Malwarebytes and Adaware are what I use (both free).