It's generally agreed that antivirus plus firewall is rock-bottom minimum security for the internet. So you'll need to add firewall protection in one way or another -- even making sure that Windows firewall is active is better than nothing.
Unfortunately free-standing freeware firewalls are getting harder and harder to find, mostly becoming incorporated into "suites" which are paid-version-only or to a greater or lesser degree bloatware (or sometimes both, of course). I used to use the PC Tools firewall on my old XP system before the system finally died of old age -- that's no longer available, thanks to Symantec's marketing policies, but I understand that the firewall in Win 7 is fairly good compared to earlier versions, especially considering my other layers of protection.
