These days, it is very hard to defect your computer if your PC is already infected. Although your security scanner may say that the problem is solved, most of the times it is still in your computer. Probably the best protection is to not let virus infect your computer first. And this means you got to have a good security program.
What I recommend:
Kaspersky Security Suite with the addition of Spyware Doctor (free edition)
Other alternatives are: NOD32 http://www.nod32.com.au/