How can I keep my pc virus-free without slowing it?

This is a question posted on Yahoo Answer by Farnaz Ahmad

I used many anti-virus soft,but none of these worked properly.I am facing some problems with explorer and file extension.In addition,those soft made pc slower.Is there any better solution without slowing the pc too much?

Steven25:
suggestion:

do disk clean up and defragmentation

disk clean up computer>hard drive c:> righ click properties>general tab> disk cleanup> tools tab> disk defrgmentation.

and use Ccleaner.

last solution: back up all data and do clean installation of your OS.

ED G:
Make sure your computer is tuned up. Most anti-virus run pretty light now days, and use way less ram than other programs. For instance I'm using Norton Internet Security and it's using 10mb of ram. While firefox is using 140 mb or ram. The only time a anti-virus should slow down your computer is when it's scanning for virus. Which you should do when not using the computer.
Now it is true that some of the anti-virus programs will slow browsing some, because of real time web protection. But not a lot, if a page takes one second to load, with some anti-virus it may take a second and a quarter. One quarter of a second isn't bad really. Gone are the days when anti-virus programs used a lot of ram and slowed down a computer. Some still do, but overall most run light, Norton, Avira, Microsoft, Nod32.