Downloads: Majority comes from downloads, so to verify the site before downloading. If require freeware/shareware programs, search safe sites like
Tucows Downloads - Download Freeware and Shareware Software or
download.com.
Agreement: Check out the EULA (End User License Agreement), which comes with terms of agreement to continue installing and often skipped, giving permission to install spyware.to unknown softwares. It is better not to accept unless sure of the software source.
PopUps: Popup messages are common and often overlooked by pressing the buttons, which might be the permission to install spyware. So, it is better to read the contents and not choose any button, but close/cancel it instead.
Virus/Firewall: Have one installed and ensure that they detect and block worms, trojans, spyware, adware etc and keep them up-to-date, like the commercial software
AdAware Pro
Scan: Possibility of infection should not be dismissed. If not have the commercial protective softwares, can go for freeware like
AdAware,
Spybot Search & Destroy, which do not monitor the PC in real time, but can be used during manual scanning for detection and removal.