Aerodynamisist,
With a PIII 500 and the amount of RAM you are running, you should be able to type without issues in the forums.
One question I did not get the answer to is the size of the Hard Drive and the amount of Free Space left. It could be if your hard drive is full, the computer is having trouble caching. If you see your Hard Drive light on when you are having lagging issues, that could be the problem.
If you think you might have a Virus, run:
Trend Micro's HouseCall
Take Care,
Richard