CCleaner - http://www.piriform.com
Run this to clean your registry and clean up disk space. When I ran it, it discovered over 100 registry errors (most related to un-installations of programs that did not completely remove the registry entries when i uninstalled the program). It also found over 2GB of space that could be saved.
Malwarebytes Anti-Malware - This program is a popular malware scanner/cleaner. This is usually more accurate in finding malware that the anti-virus programs are not as efficient at finding. The last time I ran this program was over a year ago, but I run various virus cleaning programs at least weekly, but this program still found 26 objects that were related to malware.
Lastly you might want to run a couple of Windows System Tools:
Defrag - this may not be necessary depending on the type of drive you have and software you are running. You can run it to Analyze whether your drive needs to be defragmented.
DiskCheck - This is a tool that allows you to check the physical reliability of a disk drive. This is good to run every few month.