I use Spybot and Ad-Aware SE on a regular basis when I do consulting work. Never had any problems with either of them and they always clean house. Infact, I recieved a call tonight about ad/spyware and had them download, update and run both the above programs. Problem solved.

Preventing further intrusions is an entirely different matter. Usually, as long as you do not download software by a company name you are not farmiliar with, everything is ok. For viruses, Norton AV is the industry standard. Time and time again I go to someone's house to look at a PC only to find that they have not updated or even worse, do not even have anti-virus software on their computer. Then I often have to research the virus and take it out manually