If you're worried about the constant threat of spyware and malware attacking your PC, then WinPooch packs a powerful punch.
The good thing is, it's free makes special emphasis of stopping trojans and spyware using a technique known as "API hooking" which , basically provides static protection and doesn't allow anything to lock onto your registry. Alternatively it can give you the choice over whether you want a program to write in a system directory or in the registry, or else to connect to internet. The developers are keen to point out that this is in no way an anti virus checker and as such, it's a good idea to install it with one of their sister programs, ClamWin although note that this does not provide real time protection against viruses. Overall the good thing about this program is that it leaves the decision up to you over whether a program is installed or not. The downside is that it doesn't provide a real time anti virus scanner but it's easy to use and seems to be very effective at stopping anything coming through as you get plenty of alerts.
Simple to use and powerful at what it does, this is a great anti spyware blocker but would have scored more highly if it had included an anti virus checker.