Avast antivirus protection is free and pretty good.
[link to www.avast.com]

Make sure you are also using a good firewall. Comodo is free and pretty good. [link to personalfirewall.comodo.com]

You may like to also download CCleaner, to clean your PC of temp files, run its registry cleaner, etc. This too is a free program. [link to www.piriform.com (secure)]

Hope this is helpful.