It's been my experience that scripts in the web service portion of IIS are typically run as one of the IIS user accounts. You probably need to make sure the correct user account has the proper permissions to access your database. The links below might help you figure out what the user accounts are on your IIS server.

http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/3648346f-e4f5-474b-86c7-5a86e85fa1ff.mspx?mfr=true

http://learn.iis.net/page.aspx/140/understanding-built-in-user-and-group-accounts-in-iis/