The first test is to see if IIS is serving up any pages. Place the test.html.txt file in your webmail folder. rename it, removing the .txt. Next try to load the page, for example: http://www.mydomain.com/webmail/text.html
If the page loads, it will say "Hello World"
If that works then place the test.asp.txt in that folder and rename it, removing the .txt. Next try to load the page, for example: http://www.mydomain.com/webmail/text.asp
If either these fail, then it is a IIS configuration issue.
For IIS, open the web site properties, there should be an .ASP NET tab, click it and ensure that you have a minimum version of 2.x selected.
On the Directory tab make sure it has Execute Permission set to script only (I think the installer should already set this permission).
On the actual web folder via Windows Explorer make sure that the following accounts have Read, Read & Execute and List Permissions (Security Tab):
ASP .NET Machine Account
Network Services (this is a must for .NET)
IUser_ (Internet Guest Account)
Give this a try and let us know how you made out. I used IIS6 as a reference, it has been a long while since I used IIS5, but the setup up is almost identical.
[EDIT] It appears that the forum downloads the files with a .htm file extension, so to revise te statement above, rename the files and remove the .htm extension. When done the files should be called test.html and test.asp.