How to speed up your computer, using Disc Clean up on Windows 7

1. 
    Click Start, click All Programs, click Accessories, click System Tools, and then click Disk Cleanup. If several drives are available, you might be prompted to specify which drive you want to clean.
2. When Disk Cleanup has calculated how much space you can free up, in the Disk Cleanup for dialog box, scroll through the content of the Files to delete list.
   
   
   
   
   
   3. Clear the check boxes for files that you don't want to delete, and then click OK.
      
      • For more options, such as cleaning up System Restore and Shadow copy files, under Description, click Clean up system files, and then click the More Options tab.
   4. When prompted to confirm that you want to delete the specified files, click Yes.
   After a few minutes, the process completes and the Disk Cleanup dialog box closes, leaving your computer cleaner, performing better, and potentially increasing your PC speed.
   
   Instructions taken from the Microsoft website, more tips can be found here