If the app is running only php and the database is MySQL and you really are good with Windows server I would make the switch.
While Windows would not be my choice I would not risk the site going down on a server I could not administer.
Building Apache, PHP and MySQL on Windows is very doable and for you the best choice.
A badly constructed Linux server is going to be more of a security risk then a wellput together Windows server.
Windows 2008 actually requires you to turn on the services you need.

I would test test test. PHP on Windows might need modules added that are included in a Linux distro.

__________________
"The future is here. It's just not evenly distributed.