Lets start by getting a baseline test to see what is going on in your 'puter then go from there.
First, go to http://pcpitstop.com and do the full tests. When the test results page comes up, go to the yellow box in the upper right hand corner and click the bottom link in that box. That will take you to a new page which will have a link about emailing the results. Click on that link and it will give you the URL for your test results.
Cut & paste that URL in the reply to this message so I can see the results.
I will be away from my computer for about 45 mins.
I had that same machine 4 computers ago. I loved it. When it came out, it was the ragged edge and, with it, AMD put a hurtin' on Intel. You have more than gotten your money's worth out of it.
With a hard drive that small, there isn't a whole lot we can do with it. It is currently loaded to the gills with only 78MB of free space. As a result, the speed is being hampered by the load. We will attempt to clean it up a bit here.
As for adding more security programs, your HD just doesen't have the room.
All of the other suggestions made by PC Pitstop about adding to this machine would be a waste of time and $$. So don't do any of them.
As it sits, there is no evidence of any worms or viruses and that is good.
All you can really do is use Ace Utilities to cleanup the hard drive. That will speed things up considerably.
Install it and when you get to the menu page, click on Wizard. Enable all 4 options and run the program. When it finishes dump everything it finds. Trust it totally as it will not hurt you.