In your situation, I would suggest you to use Avira antivirus. I am also using Avira antivirus free edition. I like it because it is fast, reliable, is free and won't slow down the computers when playing games. Other popular and free antivirus available are
Well there are lots of antiviruses which I liked. In my PC, I have used MSSE (Microsoft Security Essential). Here are some list of best antivirus which I like:
I have install many antivirus s/w like Norton,Avg,etc but it takes many process and in fact it slows the performance of computer and so i tell u that u should use Avast which takes only 3 or 4 process or Avira ,this two are the most awesome antivirus so try it guys .