I had a similar problem with NT 4.0/IIS4/PHP4.
The file permissions would work, then would fail. Turned out that the web server sat on a shared directory, and the share permissions were more restrictive. The more restrictive permissions over-rode the ones I set at the folder/file level.
Further, even giving IUSER_MYCOMPUTER full permissions did not solve all my problems. I had to give everyone write permissions at the share, and at the folders I wanted PHP to write to.
HTH,
Steveness