You could go the all free [no cost] route, or use a commercial solution. Many vendors now include AS[anti-spyware], integrated in their AV[anti-virus] offerings... the flagship example, being [ Norton AntiVirus 2007 ]....

Basically, if you're behind a residential router [ NAT and hardware firewall ] with Windows Firewall [also] enabled and you throw a copy of NAV 2007 on your system... you'll be fine.

With a legit copy of Windows, you could install the free [ Microsoft Windows Defender ] for AS[anti-spyware] duty and use [ AVG Anti-Virus Free ] for AV[anti-virus] duty.. that is, if you wished to avoid the cost of NAV 2007.... for a non legit copy of Windows, or to add an extra layer of AS protection, install [ Spybot Search & Destroy ] or [ AVG Anti-Spyware Free ] along with your chosen AV solution....

For "adware" specific protection, install either the no cost [ Ad-Aware SE Personal ], or the more capable [ Ad-Aware SE Plus ] if you don't mind paying for the extra protection....