Find out if they run PHP on the web server. (Yes, PHP can be run on Windows.) If so, you don't have to do anything. If they don't, then the next step is to convince them to do so. If not, then see if you really, really have to move to that server. If you do, then you'll need to work out a schedule and budget to get every PHP file rewritten in something they do support (C#.NET, ASP.NET, whatever).
Once caveat if they do support PHP: if your application depends on certain settings in the apache config file or a local .htaccess file, you may need to figure out how to do the same thing in Windows Server. Things such as URL rewriting may require this.