If you're looking for a free solution then AVG is the best, end of. The free version has spyare protection & real time link scanner. However even the free version is not as good as AVG Pro. This is reasonably priced and can be purchased in South Manchester for a tenner to blues (six grand to red twats).
NOD32 & Avast are okay, not my first choice. Free spyware protection; Windows Defender (has realtime protection). Spybot (the original and still the best), doesn't have real time protection but does have Tea Timer (kind of real time registry protection but is a pain in the arse imo). Latest versions of Ad-aware are shite so dont bother.
Thing to remeber; you can have the best virus protection in the whole fucking world but if you are torrenting or shit like that then you're gonna get virused up. Think of it like taking a stroll down the Reeperahn (Hamburg shag alley) , then shagging anything that moves without a condom then wondering why you catch aids. You have been warned.