Well. For me i think this is the best option.
1) Reformat your PC
2) Partition your HDD and dual boot Linux and Windows(Preferably windows 7 but use an older version for compatibility issues).
[Use linux for everything else and windows for playing games only]3) Boot into and patch your Windows
(Service Packs first before minor updates) using windows update.
WARNING! Do not go to any website yet as you would not have any defences at this point in time.4a) Disable Autorun(Prevents unknown infections from CDs and External removable storage like thumbdrives)
b) Disable System Restore(Prevents it from backing up any viruses if you manage to get one after)
5) Get a Antivirus + Firewall or a internet security suite and also a anti malware.
Here is a list of SOME(not all) AV, Firewalls, Anti Malware. (I will list freeware only with links and commercial ones)
(Disclaimer : It's not that i recommend them, those are the names i know and have used before. Don't really recommend AVG. Whenever it updates it crashes my SMC router (access point) and got my computer's LAN IP banned from MY OWN D-Link router (to which the SMC router is connected to) which is the main internet gateway. It happened to me but may not happen to you so...)
Antivirus(Standalone) :
AVG Free Edition 9.0(Free),
Avast! 5(Free),
AVIRA, Norton, McAfee, ESET NOD32,
Firewalls(Standalone) :
ZoneAlarm Firewall(Free)Anti Malware(Standalone) :
Malwarebytes(Free) ,
Spybot Search & Destroy(Free),
SUPERAntiSpywareInternet Security Suite :
Comodo Internet Security(Free)(Not really recommended if you are playing MMORPGs/MMOs that uses anti hack protections such as Gameguard, Hackshield,etc as those Anti-hacks will try to kill Comodo's Defense+ program(which is the part responsible for monitoring the memory) and the Defense+ will retaliate, causing BSOD(Blue screen).) Steps to take to prevent anymore viruses/trojans, etc from invading your computer.
1) Update your computer regularly(Windows updates, Antivirus, firewall, antispyware, etc)
2) Becareful of what sites you visit, especially those with "shady sounding names". However, legit sites should be watched too as there is a chance, though not high, that they are hijacked.
3) Check your HOSTS file (located at C:\WINDOWS\System32\drivers\etc) with Notepad(or any editing programs) for unauthorised redirections to suspicious websites)
4) Don't be an idiot and download files such as freegarenahack.exe that is around 100kb from sites like FreeGarenahacks.tk or any other free webhosting sites.
5) Do not modify system files unless absolutely needed or if you know what you are doing.
Sorry for this lousy guide as i'm not good with words.
EDIT: i won't write anything about linux as i don't know anything about it but i heard it is a good OS