419, inter alia, is right.
With modern high-speed disks, a big cache and newer file management schema, the old thrashing has gone and the bottleneck is occurring elsewhere.
You were not specific about where your machine is running slowly. Other posters have pointed out that you need to destroy any nasties which will encumber your Internet connexion - mainly spyware - and that's a sine qua non.
If your apps are slow, examine where the bottleneck is - is it i/o, RAM or cpu? Could it be that you are running bloatware?
I assume you are running a version of Windows. The defrag freebie is a placebo and will not even defrag your swap file (the most important part, if you haven't given it its own playground, prefereable on a separate disk altogether.)
The PC is not designed to run more slowly. Your bloatware is. Try doing a find under Windows Exploder on XP and you get a yapping dog to help you. A wonderful use of CPU cycles.
Just ask if you want to know more, giving your system's specifications and specifics about your slow programs.
BOFH