You mean like ~70% of all home computers until a year or two ago? It's routine. Has been for years.
You could plug a Windows box straight into the net safely, without third-party plug-ins, starting from the introduction of (if my memory serves) XP Service Pack 2, which was the first one with a built-in firewall.
The XP firewall is perfectly usable and always has been - if it wasn't, the whole damn planet would have melted down long before now.
(Tannin puts fingers in ears and awaits the host of brain-dead know-it-all posts from the usual suspects pointing out some trivial security issue that, in reality, has never been a factor so far as real world infections go, but which, in the hands of a brain-dead uber-geek, can be inflated into a pretend nuclear meltdown. The reality, let us remember, is that the Windows XP firewall works just fine and has done for many, many years. I have always preferred a hardware firewall in front of it, but that's just me.)