I second Spyware Doctor but I will raise you Spyware Doctor with Antivirus. It's a better solution then just SD by itself.
With that said I would disable the browser defender because it does slow down surfing and moving around.
AVG and Spybot are nice. However the free version of AVG offers no upfront active protection and the Tea Timer from SB is very limited.
Malwarebytes is very sweet but again you need to purchase the full client to get the active protection.
At the end of the day I do not care how much common sense you have. You still need a good active security client. 1000's of sites are hacked daily and malisious code is injected into them. The free client versions of AVG, SB and Malwarebytes will not do the job.
I write virus remove guides for a living and regularly infect computers all the time for testing. SDA is the best at preventing. Just my personal experience.