Your computer is still running at full speed but it's probably doing too much unnecessary work.
Use Start/Run and enter
msconfig
in the box & click OK
Select the Startup tab and uncheck all the boxes.
Download and install Spybot Search & Destroy (free) and run it - it will probaly finds hundreds of unwanted tosh in the machine.
Download and install Piriform's CCleaner and use the registry tool to tidy up the registry.
Delete any unwanted programmes using Add/Remove programs utility.
Run the Disk Deframent utility
Run Spybot & CCleaner again
Right click in the taskbar (strip along bottom of screen) and select Task Manager - see what programmes are running and which processes are eating resources.
Run the registry cleaner monthly, Spybot weekly and defragger once or twice a year.
Add more memory if it's still too slow for you.
Peccavi
July 2009