I would like to add the flip side to the free AV program MSE for your consideration. MSE does not contain a firewall. You will depend on Windows firewall. The Windows firewall blocks all incoming probes from the Internet but by default, does not block outgoing connections UNLESS you write a rule to block. In default setup, all outgoing connects that do not match a rule are allowed. In Windows firewall setup, there are no outbound "Block" rules, only "ALLOW" rules. So even if Windows firewall outbound protection is "ON", it is not doing anything. No outbound protection allow broadcast of your personal info or allows your machine to be used as a part of a botnet.
MSE does not check incomming compressed files (ZIP,RAR) Most files or programs from internet are compressed. MSE does not check e-mails only attachments when they are moved to hard drive.
MSE is the best free AV program out there but go in with your eyes open
Glenn