Advertising software and spyware are the number one reason a computer runs slower than usual. I use two programs on my computer - Lavasoft's AdAware and Spybot Search & Destroy.

And of course you need antivirus software. I use Norton and the free version of AVG Antivirus.

Another very useful program is Startup Monitor, which will tell you when a program tries to put itself in Windows Startup (an annoying problem with Quicktime).