Icrontic has a bit more detail up about Windows Preinstallation Environment.

There are two different versions of Windows PE - 32-bit and 64-bit. One is for the x86 architecture, and the other for IA64, respectively. WinPE is too large, of course, to fit on a floppy disk, but it will however fit on any other bootable media, including a CD-ROM, DVD, a RIS server, or just on a machine's hard disk. The CD image weighs in at around 120MB, but takes up a mere 40MB of memory after fully booted (x86 version). Here's a more detailed breakdown of where the space is going.