I like to recommend you avast free antivirus which can support windows 7, maybe you can have a try.
http://download.cnet.com/Avast-Free-Antivirus/3000-2239_4-10019223.html?tag=mncol;1
BTW, although antivirus software is a must, it cannot 100% protect your files. So recommend you a free backup software which can make a duplicate of your data and 100% protect your system and files. You can have a look.
http://www.todo-backup.com/products/home/free-backup-software.htm