Well, if you don't know if you've installed the Spyware Doctor, things get a bit difficult.
Go to Control panel/add or remove programs and scroll down to the Ss and see if it's there. What was not clear from the link was that the version offered would not remove anything unless you paid. So, if it's there, uninstall it and restart your machine. If it's not there, fine.
Go here instead:
http://pack.google.com/intl/en-gb/pack_install er_new.html?ciNum=11&hl=en-gb&gl=uk
Uncheck everything except spyware doctor, then follow the on-screen directions. This will install the free version.
After it's installed, click Run, and the program will open and should automatically start a scan (if it doesn't start scanning, click the Scan now button and choose a full scan. When it's finished the scan it will show you the threats and allow you to remove them.