Warning!!
Use these tools at your own risk. If you don't use them carefully, or with full knowledge of what they do, you could ruin your computer. You've been warned.



Spyware removal
Malwarebytes' Anti-Malware - Very quick and effective tool for removing malware. I recommend running this from Safe Mode, and then run it again from your regular profile.
SuperAntiSpyware - Another VERY effective tool for removing spyware.

Clean-up and maintenance
Defraggler - I find that people want to defrag their hard disk because they are told that it will make their computer faster. It won't, unless your computer is about 15 years old or older. However, if you are determined to defrag your hard drive anyway, then use this free tool. It is more effective than the tool built-in to Windows, and it is a lot more entertaining to watch.
CCleaner - Removes old temporary files, memory dumps, aging antivirus definitions and hotfix uninstallers that you'll never uninstall. You'll be amazed at how much disk space is being wasted.
Eusing Free Registry Cleaner - I don't recommend registry cleaners for most people. The risk is too great. However, if your computer has had a massive virus/spyware infestation, use this after the clean-up.

Anti-Virus
Microsoft Security Essentials - I believe that this antivirus software is better than most of the retail software out there. It is lean, meaning that it won't slow your computer down as much as the others, and it is effective. If your subscription has run-out, uninstall what you have and get this right away.