The PC Essentials

SuperAntiSpyware is an excellent and free way to get rid of and protect your computer from malware, spyware, trojan, and other crap.

Spybot Search and Destroy is clunky and sometimes freezes up, but  it's free and has found things that SuperAntiSpyware couldn't detect.

CCleaner wipes out all the temp folders and other junk.  It also has a good registry cleanup tool.

If you use Firefox, get Ad Block Plus.

Internet Explorer users, block those third party cookies.  Go to Tools \ Options \ Internet Privacy and then click on Advanced.  Select Override automatic cookie handling.  Accept first party cookies.  Block third party cookies.