Microsoft AntiSpyware is the only anti-spyware program to offer free real-time protection, comprehensively locking down a computer. The only free alternative is Winpatrol. Anybody not paying for an anti-spyware program with real-time protection (e.g. Spy Sweeper) should have MS AntiSpyware.
The latest tests suggest that even the best anti-spyware programs only find 80% of spyware, so the best approach is to use two or three scanners, scanning with each when definitions are updated.
I use all the free scanners occasionally:
Ad-Aware
Spybot Search & Destroy
Microsoft Anti-Spyware
Yahoo! Anti-Spy (From Pest Patrol)
X-Cleaner Free
A Squared (Trojan scanner but also finds spyware)
Ewido (Trojan scanner but also finds spyware)
It's important to use only one real-time protection program: from the above list, only MS Anti-Spyware has comprehensive real-time protection. (Spybot has a few lock-down features if you activate 'Tea Time'.)
ANYBODY USING AVAST! HOME SHOULD USE MS ANTI-SPYWARE BECAUSE AVAST! HOME DOES NOT BLOCK SCRIPT VIRUSES AND MS ANTI-SPYWARE HAS A SCRIPT BLOCKER. (If you're not already using a script blocker such as that from Analogue-X , of course.)