Need to install a freeware antivirus yourself?
Here’s a simple step-by-step guide from Life Hacker. You will need more than an antivirus software to completely protect your system but if you have to do it yourself, this should see you through getting the most crucial software component installed. The recommendation is BitFender for Windows. Make sure it is the Free Edition. For the advanced user, Avast is the favourite choice.
I have personally been using Avast for as long as I can remember. Why? I have no reason to look for another option, LOL!
As advised, remember to have good and safe browsing habits. Prevention is better than Cure!