I deal with malware, viruses, and botnets on a daily basis for work. I also employ exploit researchers whose job is to look for and write exploit payloads for holes in applications, protocols, etc. I collect "in the wild" viruses as a hobby to analyze the code.
If you have any questions about viruses or malware, please let me know. Earlier recommendations of AVG, Spybot S&D, and CCleaner were spot on. They are some of the best free utilities you can get at this time for home users... though I would suggest AVAST! rather than AVG currently. If you can, install them on a CD-R and run them from there in Safe Mode on Windows. Most malware & viruses are written to bypass anti-virus apps or are AV-aware. Running them from a non-writable source like a CD will prevent an infected machine from disabling or avoiding your detection & cleaning engines.