my computer is slow

i dont know why but the computer im using now ( i alternate between three different computer) is running very slow. i gnored it for the most part but now its causing my browser to run at half what it should be doing and causing it to not respond atleast 3 times when i log on. I know its not a virus because the other users on this computer (my sisters) run slightly faster than mine with almost no problems.
help would be appricieted
help would be appricieted