Lots of older computers have a problem with memory. I am fixing a computer right now that had malware on it and only a half GB of ram. This computer was running a full antivirus suite from McAfee though the only problem is the computer was incredibly slow.
I mean really really slow. The OS was windows xp and the OS it self was taking up ~200 MB just to run the OS and on a default start up with McAfee running it was running around ~600MB which is a bad thing. McAfee was taking up almost the whole ram that was on the computer leaving nothing to be used by the computer OS or any application.
So after uninstalling McAfee the computer ran a lot faster probably 10X as fast. But the only problem is the computer did not have any anti virus software anymore. So I looked for a alternative antivirus that offered real time protection. What I cam with is clamwin this uses a lot less ram than Immunet 3.0 that you get from clamav.net which used 200MB of ram so still to much for a computer with only a half GB of ram in them. The only problem is that clamwin does not provide real time protection. So I needed to fix that to help pro actively prevent viruses.
So the solution is Clam Sentinel. This solution is not the best seeing that the clam database is not the most complete but it is the only low memory solution that is fitting for low memory computer. Running clamwin and clam sentinel only uses ~20mb of ram this helps the computer be able to do other things like running firefox with out having to use the hard drive as virtual ram which slows the computer down a lot.
Hope this Helps some,
Local Tech Repair Admin