We're constantly being asked for recommendations for free antivirus software. The most popular among ABers (including me) appears to be Avast:
https://www.avast.com/en-gb/free-antivirus-download
BTW, upgrading to Windows 7 needn't cost you an arm and a leg. You can buy a Windows 7 desktop PC (which is perfectly adequate for web-browsing, video streaming, image editing, word processing, DTP, etc) for just £59 here:
http://www.accomputerwarehouse.com/products/computing/refurbished-computers
(That's where I get my computers from).