nt 4.0 + iis 4.0 + asp files

We are getting a peculiar bug. We have NT 4.0 running on the webserver, along with IIS 4.0. We can execute an .asp file from the server, but NOT from any other machine on the network (the browser display an http 500: internal server error messge).
On IIS 4.0 we have the home directory /permissions set to execute.
What could possibly cause this?
Thanks.