IIS and form submission trouble with PHP site

I'm running a php site in IIS. the index.php loads fine but when I try to go submit a form to a script I get a 500 internals server error and when I look in the .NET panel of firefox an encoded 开发者_运维技巧forard slash (%5c) has been added to the end of the url that the form submits to.

I have the required dll's working so IIS works with PHP.

I'm not experienced in IIS. Have googled this to death. Any ideas?