It has everything to do with anti-trust suits. That is why in Europe they have to give customers the option of what browser they want to install and not putting IE, WMP or other programs on. Kind of ridiculous IMO. I understand they own 90% market share, but people drool over the fact that OSX comes with different programs built in and they don't have to go download anything else. They all of the sudden think that they can't get these things on Windows, but little do they know they can install Windows Live, or tons of other software, but MS can't put it in because they will essentially take out any AV software developers or developers for other programs.
/rantover
Anyways these are the programs I use to clean up peoples computers (all the free versions which work great)
Malwarebytes : Free anti-malware, anti-virus and spyware removal download (free version)
SUPERAntiSpyware.com | Remove Malware | Remove Spyware - AntiMalware, AntiSpyware, AntiAdware! (free version)
CCleaner - Optimization and Cleaning - Free Download (free version)
Virus, Spyware & Malware Protection | Microsoft Security Essentials (free version)