If it is for home use, I suggest I use two I am reasonably happy with: Trend Micro PC-cillin 2005 (http://www.trendmicro.com/en/home/us/personal.htm) and lavasoft Ad-Aware (make sure you configure it well). If it is for corporate use, Trend micro had client server SMB software as well. Very usefull, centralized management, and so on. Test it first (download and install, or use Housecall (free scan):
It works quite well for me. Whenever I notice a significant slow down of my PC performance, it is rarely installed spyware – but most often it is cookies. And, by all means, do not use freeware – Shareware is tricky – but the ones you have tested and have confidence with.
Hope this will help you.