I use Ad-Aware (it's free - just do a google search) and I also like Spybot
Be careful clearing your cookies too much though. I am not sure if this has anything to do with it, but I was recently NOT credit for two shopping orders (one being a pretty big one). After corresponding with Tricia, I think it was because I clear my cookies TOO much that I didn't get credited. I use CCleaner to do this and it clears all your tracks - cookies, temporary internet files...everything!
I was originally told that I need to clear my cookies in between doing each offer, but there have been a couples of times that I have done that and when I went back to SR, I was logged out.
All hope is not lost, I still may be able to get the credit, but I thought it was important to pass along. Come to think of it, that may be why I never got credit for a lot of the smaller offers I did in the beginning...I think I'll go back re-do them...