Asp Create Mailbox Question

I've created an ASP web page where users in our organization can create
Active Directory computer accounts.

The web page is running on a Server 2003 SP1 IIS 6 installation. The
Exchange System Manager is running on the web server and Exchange SP2 has
been installed. The IIS site is configured with Basic Authentication and
users are prompted to enter their Active Directory credentials when
connecting to the site.

The web page uses ADSI code to create a user, and then CDO code to create
the mailbox e.g.

set oOU=GetObject(<LDAP string for OU>)
set oUser=oOU.CreateUser("user",<CN for user>)
' set a bunch of attributes
' now create mailbox
set oMailbox=oUser
oMailbox.CreateMailbox <string representing MTA>

Now, when I connect to this site using my domain admin credentials, it
works. However, if a user connects to this site, after being prompted by
IIS' basic authentication for AD credentials, the user account is
successfully created in AD but upon reaching the "CreateMailbox" line, I get

Microsoft VBScript runtime error '800a01b6'
Object doesn't support this property or method

Thinking that this was most likely a permissions issue, I took the .asp page
containing the code and converted it to a .vbs file. Then, I logged in as the
user account and used cscript to execute the .vbs file. Both the user account
and the mailbox were created succesfully.

I don't understand why the code runs under the context of a user account as
a vbs script, but doesn't run as an ASP page unless the user is a domain
admin. It's not permissions on AD or Exchange or the code wouldn't have run
as a vbs script. What is different about running it as an ASP page?