These are the free ways to protect your Windows computer that I especially use and recommend; the links are up to date as at February 2014. However, websites change - please let me know if you have troubles.
I’d be pleased to hear how you get on; email webster@theoldie.co.uk or click here
In the past I have recommended ad-aware - but I have now withdrawn that recommendation. The latest version seems to be messing up computers royally, and so it is best avoided, at least for the time being. They have also made it more difficult to get the free version, which is irritating.