Sorry but this view that Linux is by default more secure than Windows is wrong. Does GnuTLS mean anything to anyone? How about all of the issues in OpenSSL recently? Vulnerabilities affect Linux just as much as Windows. More so in some cases.
In general, as long as you are keeping your OS and applications patched, you are doing the first few things right. Anti-virus and anti-malware help too, as would a software firewall on the machine.
If you want to go into ultra paranoid mode, then have a look at something like Sophos UTM, or purchase a hardware firewall. Ensure that any ports you don't use are blocked both inbound and outbound.
Use something like Norton DNS so that you don't go to known phishing and Malware sites. Install NoScript in your browser and whitelist only sites you trust.
Ensure you have a sound backup strategy with redundant backups and off-site backups.
Just a few things off the top of my head, but this idea that Linux is inherently more secure than Windows is a myth