Why is my browser redirecting search results to fake search engines?

My virus protection was malfunctioning for a short time yesterday, and during that time I got a virus. I believe I've gotten rid of it, but I'm now experiencing another problem, which I suspect is some sort of malware that got onto my computer at the same time as the other virus.

Whenever I try to do a search (mainly just searches pertaining to the problem at hand, although some normal searches as well), any link that comes up redirects me to a site to buy things or page of fake search engine results. I'm not sure what's going on and I can't find out because whatever has installed itself on my computer or in my browser is hijacking my search results. I'm sure there's an answer out there, but I can't find it because I can't do a proper search. Any tips you can give me for how to eliminate this problem would be greatly appreciated.

I use Firefox and XP Professional. Also, I have been receiving a lot of notifications from Nortons that an IP address in Luxembourg (212.117.174.176) is trying to attack my computer. I'm not sure if this has something to do with it or not.

Thanks.