> c - how to stop it happening again
I suggest running a program like WinPatrol. Get it for free from here
http://www.winpatrol.com/
After you install it then it runs all the time in the background and it monitors your Windows system. It informs you of any new program that tries to install itself, or if anyone tries to change your internet settings.
You get the chance to say NO and a program is not installed.
You can also look at many Windows files and programs and sort them by date to see if any have been aded recently that you dont know about.
For example with WinPatrol you can check StartUp programs and see if any have been added recently, and also hidden files that may have been added recently.
WinPatrol is an excellent program that everyone should have running on their computer.