It will require you to reinstall most of your programs. Use this only if you are an experienced Windows user, you have a free day or two to reinstall everything, and you have the programs to install. If you install and uninstall a lot of programs, your Windows directory will become huge. Deleting the Windows directory will remove these extra files. This was suggested to me by a Russian programmer.
Russian programmers are a tough sort.
It is a crude way of getting the job done, but it works. It is a good idea to do this once a year or so. You do this by typing deltree , a space, the location of the Windows directory, and Enter. Warning - this will erase everything in the specified directory and all of its subdirectories. Use the deltree command with caution.