Well depending on what is happening with the other site you may find something work different and some functions not working at all.
As for changes in scripts probably the only obvious thing is database pass/user and database name combination.
I dont recommend using PHP on the windows platform, however if that is what you want to do it wont be as secure and like i said somethings will work different and some may not work at all.
As for installing you will need the zip from php.net copy it to c:\php
There is a install.txt which is fairly straight forward with the installation. You will need to install it as an ISAPI module, as installign as a CGI on IIS will alert you with a security warning.
If you have trouble with the installation i can help.