This website has a very good summary.  It’s a little old now though – so I read it at the same time as the ‘Program Portal’ instructions (using the latter, when possible).

This is a really useful free app for getting UDID’s from your users (beta testers, translaters, whatever).

To create the distribution ad-hoc build, follow the instructions in the Developer Portal.

To make a windows compatiable ad-hoc build version, drag the application bundle into iTunes, then create a zip from the IPA that that action creates.  I use iTunes on a separate user account to avoid polluting my main one and mixing the dev and ad-hoc builds.  Then give your windows users the IPA.  The Windows iTunes is a lot less forgiving and can’t load the bundles directly.  More tips are here.