I would recommend
SOPHOS.
They have an anti-rootkit program and a free anti-virus scanner which can be used even if you already have an anti-virus installed on your computer.
Despite good anti-virus you might also need anti-spam software and these are the ones I've tested and which have worked good:
CounterSpy
Web Root Spy Sweeper
Ad-Aware 2007
AntiSpy 2.13
SpyHunter
Spyware Doctor
CounterSpy is my favourite for realtime scanning, works good when you play games at the same time. No big slowdown.
Spy Hunter works really great and I've personally bought a registered key and I can say that it's able to detect things that my other programs can't. Some people has said that Spy Hunter is no good compared to other anti-spyware agents. Don't listen to them! :-)
Ad-Aware 2007 works good.
AntiSpy is good if you want to clean cookies in your browsers but cannot undo what it removes, unfortunately, so it's a bit dangerous but removes things efficiently.
Spyware Doctor also is to recommend.
By registering you get scheduled scanning possibilites which is great to turn on while you are sleeping at night. For instance Spy Hunter takes several hours on my system and when I wake up in the morning I can see the report of the scanning process.
F-Secure has good anti-virus and I use this in combination with Sophos Anti-Virus check tool and sometimes I use online virus scanners which I can link to if it sounds interesting.
I personally is a bit paranoid when it comes to computer virus and malware and stuff and because of this have lots of programs which deals with this stuff. One cannot be too careful these days. Keyloggers and password sniffers is 2 examples where CounterSpy is very eager and competent to deal with, thought I would mention that too.
So, as I was saying at the top: test Sophos free products and see if it can find anything. Highly recommended!