Soon after I installed Calibre and I saw Windows PE in debug's Windows info I posted a query about it. Someone, probably Kovid, told me calibre gets that information from Trolltechs Qt package. Calibre reports Windows PE, on my home brew systems and on my Dell Alien. IIRC Speccy differentiates by slapping 'OEM' after the license number on the Alien - currently on loan so I can't check.
My knowledge of this is a bit rusty but I think the following still holds true:
No matter what the status of the Windows licensee, OEM or Retail, the first thing that will be installed is the 32bit Pre-installation Environment.
In the case of Retail the install process will then automatically and immediately proceed to installing the relevant full version of Windows.
If all you had was 32bit Windows PE you probably couldn't even install calibre, because amongst other things there's no Trusted Installer - msiexec and all that.
BR