Once you have checked for spyware and run the PC Decrapifier, and your computer is still running slow, it is time to take a good look at the programs you have running all the time. Some you can see in your system tray. Others run invisibly in the background, hogging system resources and slowing down your machine.
To help, there are two great Windows utilities, Autoruns from Microsoft and the Startup Control Panel from Mike Lins. Both do much the same thing. They show your computer runs, whether at startup, logon, when you start Internet Explorer, etc. Just uncheck the services you don’t want running all the time. Like Quicktime, RealPlayer, and other stuff. When was the last time you actually used Bluetooth? Shut it down. What about infrared for file transfers? Shut it down.
Prune the services you don’t need, and you will regain a lot of speed.