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.
sounds like many of the Dell / HP workstations you find in offices. With more RAM you should be more than able to run the applications you use. Check the mobo manufacturers website for maximum memory it supports then upgrade. Ok the CPU is an old design but they are more than capable to run office applications