was it running fine for what you use it for before adding antivirus? If so, and it works for what you need it too then just add 512MB of ram and you should be back to running how it used to. Also check to see if the hdd is fragged. But to quote werxen
dude your cpu is ancient
the system is quite dated, but if you need to limp it along adding that ram should getcha back to where you were.