Heres my suggestion.
1. If you have programs like Music Match or Windows Messinger hidden on your system tray, RIGHT CLICK on them, and go to exit or close. I wouldnt reccomend doing that to a virus protection program.
2. Run RuneScape in low detail.
3. This actually worked for me, I had a very slow computer, so I downloaded the offical client from the Jagex website. It ran a little slower.
4. If you are playing in a f2f world, then you can resize your Internet Explorer window and adjust it so you will not see the advert at the top of the page. Sometimes, the adverts can be whats slowing down your computer so much
5. This is a little harder, Go to the control panel, then choose add/remove programs, and remove any unwanted software.
HOPE THIS HELPS!!!