Your IUSR_* account needs permission to the instantiate the COM object.
Or you need access to someone who does have permission. I've seen a scheme based on having the web server queue files to another process. The second process ran under a different (more privileged) user account, and had full access to COM and DCOM. This split was some for real or perceived security reasons. (It's been a couple of years, and I've lost the details.)
A benefit of staging things this way is that you get a complete, file-based history for debugging.