Step 1: Procedure
If your computer has 2+ user accounts, and it has not been reformatted for an extended period of time, you are probably better off reformatting the computer and starting fresh. Most computers have a built-in destructive recovery. If you choose to start by reformatting, backup all your files and reformat the computer, preferably with the built in recovery tool.
Alternatively, you can create another user account with administrative abilities. Then login with your new account and delete the old account. This essentially gives you a fresh computer because most slowdowns are caused by a bogged down registry from extended periods of use. The new account has a fresh registry and is like new.