Why get involved with trial versions, etc.
For Seven probably the best free Anti Virus are Microsoft Security Essentials and Avast.
Both are free and rated very highly even against the paid Anti Virus. I have used Security Essentials for 2 years and never caught a virus. I am not trying Avast to see which I prefer. Avast resquires you to give your email address at sign up and every year thereafter, but its still free.
I made an email account on Yahoo, thats used for purposes just like this. I like free but dont like spam.