You may be running into an IIS 5 bug:
http://support.microsoft.com/kb/216493
It drove me nuts for more than 5 hours! XP Pro uses IIS 5.1
and if you try to to a POST to a URL that ends with a "/" (no page
indicated), then it won't work. We do that in our system and let
the default page (index.cfm) be picked up.
The issue was fixed in IIS 6, but you can't run that on XP.
The only solution I found was to uninstall IIS and install Apache,
which has worked great. Not only is Apache light, but I can run
mulitple Web sites with it (IIS on XP limits you to 1).
I hope that helps.