According to their website:
Pro Tools software and hardware are currently not compatible with any version of Microsoft's new Windows Vista operating system. For the latest information, check the announcement regarding upcoming support:
I think most new PCs come with Vista now, so unless you want to reinstall the OS, you should go with a MAC.
(Of course, if it were me, I'd buy a PC, install Linux, and use Ardour, the FREE equivalent of ProTools).