[W.]

Malwarebytes comes well-recommended and is free. With Windows firewall, MSE (Microsoft Security Essentials) and running MWB weekly and also using MWB to scan downloads, I've had no problems and paid nothing. :)

[Leto]

Malwarebyes bytes is good and free,

I have also used CCleaner Glary utilities both are free, alll the above can be downloaded from Cnet:

http://download.cnet.com/windows/

[TurboZamboni]

I have that as well.

[Jack Hydrazine]

Super anti-spyware is a really good one, too! And you can get it free.

http://www.superantispyware.com/