How can I transfer iPhone + iPad data to a new computer? I have iTunes installed on my old computer (Win XP) and all iPhone + iPad apps + all backups are done on this computer.
Now I would like to use iTunes on my new computer (Win 7) and retire the old one.

My experiments so far went really bad. If I do the obvious thing and install iTunes on the new computer, then try to sync the iPhone, it completely wipes the iPhone and all app data. That happens even though the iTunes account is the same as on the old computer.

My goal is simple: I want to be able to sync/backup both iPhone and iPad on the new computer without having to completely install all apps again and loosing all the app data on the devices.

This seems to be such an obvious thing - to switch to a new computer - am I just not seeing the easy way to do this?
There's an Apple guide to this process  

There's also this guide on HowToGeek.com