A useful program to run is Winpatrol (free).
http://www.winpatrol.com/
This is a small programs that runs on your computer ALL THE TIME.
If anybody tries to install a new program, or update your browser settings, or do anything else without your knowledge, then WinPatrol will ask if you are happy with that.
You have the option to say No and it will stop the installation and delete the files.
The problem with most anti spyware programs is that you run them AFTER you have got the problem, WinPatrol is running all the time, so it often stops the problem before it gets hold.
(Some anti spyware programs do run in "real time" meaning they are monitoring your PC all the time looking out for spyware programs, but not all do)