Spyware Doctor has been around for some time now and has consistently been in A-list for its effectiveness.
Recently PC Tools released the latest version of its product (SD version 5)click here and it seems this version causes considerable problems (PC freeze, apps not working..etc.)
Fierce debate rage about this version click here
I tried SD 5 and had to uninstall it after frustrating hours spend to make it work. Went back to version 4, which is what an anti-spyware product should be. active scanning, browser guard shield and more.
I would recommend version 4 but NOT version 5.