by Mauno Korpelainen.
You might need that IIS_USER or IUSER - what ever it is called...
If you download from http://www.bfcnetworks.com/whitepapers/installing-moodle-2-on-windows-server-mysql-php-and-iis7/ and there link Click to download PDF a pdf document you should find from page 10 some info about setting up IUSER account.
I am not sure if this alone explains your case but at least it sounds like an IIS permission issue...