WinPE stands for Windows PreExecution Environment and is part of downloadable Win 7 software, the WAIK (Windows Automated Installation Kit).
For much, much more info on customizing winPE and related topics. I recommend Reboot.Pro: I also recommend that you sign up for their RSS feed and their newsletter.
You will be interested in WinPEBuilder
The reboot.;pro forum is the best place for you to ask winpe questions.
We only scratch the surface here, although we have a couple of very versed members.