File Upload Does Not Work

I'm using FileUPload from softartisans (not my choice, but my company's), and in our testing environment, the anyonymous web user can successfully upload and delete files to the server (iis 5.0/win2000).
However, in our production environment (same software/hardware setups), the web user can only delete a file, the file upload doesn't work. I've set if Err's do a response.write after each and ever step...and no errors are written out. i can do a generic response.write("Test") at the top, middle and bottom of the script and it prints on the page. The IUSR_xxxxxxx internet guest account has modify access for the directories (identical to the testing environment).