Problem debugging asp code on iis 5.1, xp pro

I am trying to step through asp code that I have written using .net
2.0, written using visual studio 2005, on iis 5.1, running on xp pro.
When I try to run the project from VS using F5 (run with
debugging) I get the following error...

"Unable to start debugging on the web server. Unable to connect to the
web server. Verify that the web server is running and that incoming
HTTP requests are not blocked by a firewall"


Why is this occuring and what needs to be done to correct it?

The project runs correctly if I run it using ctrl F5 (no debugging).

The project is using anonymous access on iis and is using the default
user account that iis gives it.