Uploading of large files

I tried to upload files to the server. Everything works fine with small files but large files (about >500KB) doesn't reach the server. They seems to be uploaded (I have to wait...) but the script in the "form-action-script" is not executed. The explorer says nothing, but showing an empty screen containing the result of the action-script (of course empty, since the script isn't executed), the navigator tells me something about "the file seem to contain no data". Enviroment: PHP 3.15, IIS4, WIN NT4, PIII, 128 MB etc. Any idea? Thanks. Kai