Below is a list of numerous programs that I've had great luck with. Many of them are award winners in their respective class.

Starting with antivirus software, my favorite, and the favorite of many in the industry is Avira Antvir. The free version is excellent, but the full premium version is definitely worth the yearly subscription.

http://www.free-av.com/en/download/1/avira_antivir_personal__free_antivirus.html

Keeping this simple, I'll go on to different utilities, give a summation, then the link.

The best defrag I've used is IObit Smart Defrag. It's a defrag and optimizer all in one.

http://www.iobit.com/iobitsmartdefrag.html

OIbit also offers Advanced System Care. Please note: This program doesn't always work properly on the PCs that I've installed it on using Vista as the operating system. Please note, too: The full version has a disk cleaner utility. ALWAYS back up everything if you use that utility! The backup is an option when the scan is finished.

http://www.iobit.com/advancedwindowscareper.html?Str=download

Eusing's free Registry Cleaner is a nice program. It's relatively small and pretty fast.

http://www.eusing.com/free_registry_cleaner/Reg_Cleaner_Features.htm