I have tried Nod, AVG, Norton & Kaspersky. But personally, I like BitDefender. But I don't recommend it to you guys, unless you have a QuadCore Processor.
ONLY AVG free is light one. Others eat lot of memory.
Digit, Again you are thinking wrong. Check this sites:
*www.thepcspy.com/articles/other/what_really_slows_windows_down/5
*www.virus.gr/english/fullxml/default.asp?id=82
*hcp.wincustomize.com/articles.aspx?aid=116286&c=1
*www.pcworld.com/article/id,124475-page,1/article.html
Play with Kaspersky or Nod, both are better. If you need a professional solution, bitdefender is the best.