Actually, WinPE is available to everyone, even those without a Vista license.
Technically, you have to download the free (4GB?) WAIK (windows automated installation kit) and use it to create a WinPE image (a WIM file).
However, since there isn't a licensing requirement per-say (i.e. no serial, no activation, and very loose to no redistribution limitations) people have created pre-made WinPE images available for download as recovery discs and WIM files.
You can actually add an entry to the Vista bootloader with EasyBCD to do some really neat stuff with WinPE images - such as adding an entry to the recovery center in the bootloader (no need to boot from the CD then), installing Vista without a DVD drive (since the whole Vista installation procedure is but a WIM image in the first place), and other neat things...
You can even boot off a networked WIM image if there was a need to do so..
WinPE gave me a lot of trouble to get it working esp. since out-of-the-box the Vista bootloader is not capable of booting a WIM image... but it's quite a nifty feature!
WinPE 2.0 is lightning fast..