Paranoia can really waste a lot of your time.
Unless you want to make a career of managing security on your workstation the simplest thing you can do is buy a 50-100 hardware firewall - netgear and linksys make these - and invest about two hours of your time to set it up and become familiar with how to manage it.
The other things are also simple: build your system once and then back it up, and then lock it down - do not install any other software.
Also, use your system ONLY for trading. Use another box for other activities and dont let that box on the same network segment as your trading system- you can achieve this by setting up a VLAN using two routers - OK this is a bit more complicated but still pretty simple.
Microsoft has free material on their website that shows how to secure workstations and servers - if you use microsoft OS its worth a read .... Basically the hardware router will stop most problems but will not stop phishing attacks or email attachment viruses. Its up to you to not enable these ... although we stop this by not allowing attachments or HTML mail in our networks except in special whitelisted cases.
