I use AVG Internet Security and find that it does not impact my viewing in a significant way (except when it does a full scan -- but I only set that off manually when the HTPC is idle). AVG takes care of my Firewall, Anti-Virus, Spam blocker, Spyware blocker, etc.
I have another PC upstairs for email etc. and since both PCs are on my home LAN, I log into the machine upstairs using Remote Desktop Connection whenever I want to check email etc. from the HTPC.
MS includes a firewall with their OSes. Firefox includes a popup blocker. MS has a malicious software removal tool for free that you can scan every now and then with. As long as you aren't downloading random programs and running them you should be fine enough.
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