This is not as simple a question as it might seem. It depends on a) how your router is configured and b) what you are trying to protect against.
Most home routers are configured to use NAT, with pretty much no port forwarding. This means that you tend to be pretty well protected from 'inbound' nastiness by default. So, if you want protecting from outside stuff probing your PC, then your router firewall might be enough.
The flip side of this is 'inside stuff' getting out. This is the additional protrection offered by 'personal firewalls' as they tend to alert you if something tries to get out - you then get to choose to allow it or not. Therefore if some nasty Trojan gets through and tries to connect back to home, you have some opportunity to prevent it from doing so. That said, the Windows Firewall doesn't provide that, so really it adds very little if you have a reasonably secure router setup.
Generally, a personal firewall is recommended - my work systems have a Symantec Client Security corporate package, and at home I've used ZoneAlarm and Sygate Personal Firewall in the past.
Hope that helps
Ruz
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ZeN Home 2000 - Zyxel 652
Posts are accurate to the best of my
knowledge. I speak only for myself and
for no other entity
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