ASP.NET Application Extension Mapping Disabled

[Leo]

Hi, I'm running Windows XP professinal and have an
application extension mapping in IIS that maps .xyz to
aspnet_isapi. Everything works fine until I did some
Windows Components Updates and installed those critical
updates. Now I can not chaned or add any application
extension mapping. When I try to change the .xyz
extension in the Add/Edit Application Extension Mapping
window, the extension input field is disabled. And when I
try to add a new extension mapping, the OK button is
disabled.
Wonder what is going on and how do I add a new extension
mapping in IIS after those "critical security updates"
are applied?
I'm in trouble now, because when I develop the project, I
used .xyz, but in production we want to some another
extension.
Thanks a lot in advance.