I have an iPhone 4 and I'm a windows 7 user; I have the latest iTunes installed on my PC and synchronized with my iPhone; I don't have any music or video in there, as I don't use the iPhone for that purpose, but I have quite a big app library.
If I'm going to reinstall Windows on my computer, or even move to a different one, what should I do to keep my iTunes library? I'm assuming the App Store keeps track of what I've purchased, but how to avoid re-downloading everything again?
How to avoid re-downloading everything again?
Your apps by default are downloaded to the location 'C:\Users\username\Music\iTunes\iTunes Media\Mobile Applications'. Take a copy of the apps from this location and back it up on an external hard drive, flash drive, DVD or a different machine.
When you re-install or move to a new machine, simply bring the apps back to the new PC and drag-drop them on to the apps section of iTunes.
Just backup your whole iTunes Library folder. On the new OS, install iTunes, copy your Library to the old place and open iTunes holding down Shift (Windows) or ⌥ (Mac OS).
A dialog should appear:
Select Choose Library... and select your old Library (I think you have to select the xml-file inside the Library folder) and everything should be as it has been before.
Source: How to open an alternate iTunes Library file or create a new one
