500 error on uploading large files

Question

It is a windows server and I am getting 500 error when uploading large files using PHP

I have

ini_set('upload_max_filesize', '100M');
ini_set('post_max_size', '120M');
ini_set('memory_limit', '128M');
ini_set('max_execution_time', 0);
ini_set('max_input_time', 0);
set_time_limit(0).

at the beginning of the page but it is still failing and I am stumped. Is there anything else I need to do? I have run out of ideas. Thanks