Another program I like to run, which is also free, is WinPatrol.
You can get it from here
http://www.winpatrol.com/
Once WinPatrol is installed it starts up every time Windows starts up.
It then sits in the background and monitors your PC to make sure no one is changing anything.
So if someone tries to install a program (maybe a virus or spyware) it alerts you.
If someone tries to update your browser home page, or make other changes to your browser, it alerts you.
WinPatrol also lists all your start up programs so you can see what is starting when Windows starts.
It also lists all the running programs so you can actually see what is running on your computer.
It is very small but I find it very useful.