If you are going to run a server OS, go ahead and get windows 2003 server.
IIS 6 is a truly enterprise class wbeserver, and is very rugged and secure.
Don't listen to the zealots preaching about uptime -
The only time I have downtime is when I have to reboot after a patch, which is just as often as I need to do it for Solaris 9: Not bloody often.
If you can't afford server OS, then windows XP will work just fine.
The version of IIS included is acceptable.
I run apache on windows, apache on soalris, apache on linux, IIS on 2000 Server, IIS on 2003 server, and i used to run IIS on NT4.
I've got a good basis for comparison, and which one I'd pick to use will almost always come down to what it will be doing - and excepting IIS on NT4, I can use any of them to do just about anything (excluding asp on the *nix stuff).
So, for your purposes, if you don't have a copy of win2003 Server, or windows 2000 server, just use the included IIS in windows XP.
If you need help, feel free to PM me.