Alright, if you don't want to redownload apps, go to C:\Users\username\AppData\Local\Microsoft\Zune\Applications and there you'll see a whole bunch of .zcp files. Those are all of the apps in your collection. You can move them someplace else and copy them back, if you want. ALSO, to copy them back over, you don't need to put them in the right folder, you can just drag them into the software like you do with music or other media you add to your collection. Though, if you have a limit on how much data you can transfer over your internet connection, reinstalling it may use a decent amount of data as well...