Some anti virus software can use up a lot of system resources and noticeably slow down a computer. The newer and faster computers don't suffer from this problem as much, but on an older computer like your's it can be a problem. I've used Avira (free) on older computers without a problem:
I don't usually recommend Microsoft products, but Microsoft Security Essentials is not only a very good product and free, but it runs well on older, slower computers:
AVG is a good free anti virus also, but it sometimes can slow down a computer a bit. I'm running it on some Netbooks with an Atom 1.6GHz. processor and it does OK. Only way to find out is to try it:
Remember that you should only run _one_ anti virus software at a time, so don't forget to un-install the existing anti virus first.
If you purchased an anti virus product and you're stuck with it, this tip may help you speed up your computer:
Also, your may be able to increase your performance with a memory upgrade. If your not sure how much or what kind of memory that you have, download and run this:
Scroll down to the SIW Without Installer (English-Only) version. After it's running click on "memory" in the left margin.