Andy's Quick Parasite Test
Online test for many parasites that could be on your computer

Spybot website - Download ver. 1.4 (local copy)
Great for detecting, removing, and blocking. Totally freeware.

AdAware website - Download build 1.06 (local copy)
Another good anti-spyware product, free and paid versions.

CounterSpy website - Download from Sunbelt Software
Top rated commercial product. 15 day trial, $19.95 per year. Same database as Microsoft's product, but stricter removal rules.

Webroot Spy Sweeper
Effective tool for detecting & removing spyware. Free online scan. $29.95/year.

Pest Patrol - Pest Patrol Research Center
Another effective tool for detecting & removing spyware. Free online scan. $39.95 includes updates for a year.

Microsoft AntiSpyware - Download from Microsoft
Helps protect your computer from spyware and other potentially unwanted software. Beta version is still free. (Validation required.)