Looking to give up a free, reliable, high performance solution that works on minimal hardware and pay a fortune for something that needs more hardware, is lower performance and did I say less reliable? (IMHO. And yes, I have real life experience with these ;-)
Okay, it's your call. If you want a Windows NAT and/or proxy solution, here is a decent listing. I don't prefer any one of these tools over another, especially since I haven't used them before.
Firesock (from the makers of Trumpet Winsock)
DOS program designed to run on 286+ class computers
requires another box like Linux MASQ
Requires Windows NT Server
Windows 95/98/NT compatible
Roughly $25 for Win9x and $47 for Win9x and WinNT
Costs roughly $30 for 2-3 IPs
Lastly, do a web search on "MS Proxy Server", "Wingate", "WinProxy", or goto www.download.com. And definitely DON'T tell anyone that we sent you.