Instead of getting firewall software, I would invest in a hardware firewall that is built into a cable modem router.

I agree, and can't stress this enough. A hardware NAT/Router is so much better than firewall software, and they're not very expensive these days.

My feeling is: If the machine is exposed enough to be getting those WinPopup messages, then there's a high likelihood that it has other things exposed that would allow a remote compromise. Probably hacked to the gills by now. Noticed any new FTP server folders full of porn lately?