1st of all if you are using windows server 2003 why don't you just use IIS?
at any rate you can do it the way berry showed you OR where ever you purchased your domain go into the DNS records for
(www)or(root domain)[i'd do both]
http:// is root and http://www
and tell it to point it to an IP when you do this type in xxx.xxx.xxx.xxx:81 you can put whatever port after the colon just make sure its not used by another protocol THEN go into your router and tell it to look out for traffic from port 81 and forward it to my webserver
(also making sure the FW on the server is open to traffic on that port)
if your ISP is really hardcore about you not running a webserver they'll wonder why you are getting alot of incoming HTTP to your ipaddress.