Motion Computing Reinstall Itunes
Robinson is kind of on track with what you need to do. Start by making a backup, iCloud is probably easiest so you don't have to move the backup between the two PCs. Once you have your backup done, plug the phone in to the new PC and choose to restore it, once the restore process is done you should be asked, both in iTunes and on the device if you want to set it up as a new device or set it up like an existing one using a backup. Pick your backup and let it do its thing and your device will the be just like before (when all apps and such have been downloaded) but will now sync with the new PC. @B0HICAH: When you plug in your Apple device to your new computer that's synced with your old computer, you probably get the 'erase and sync' message. Library Tunes will discover your old computer's iTunes Library ID (from your Apple Device) and apply this ID to your new computer's library.
By doing this, you should be able to put new media files on your Apple device using your new computer. You should also be able to transfer purchases from your Apple device to your new computer's iTunes library. For non-purchases, you can use a some free progarm.
Library Tunes will create a backup of your new computer's library just in case. There's a chance there's nothing in your new computer's iTunes library yet, so nothing to lose. Again, I have to mention by advocating the use of Library Tunes, although it is a free program, I may receive compensation. Or you can simply use the to help you get the work done.
There are full instructions about how to without data loss. I hope these can help you!! Just as you dragged and dropped Music, Movies, etc, drag and drop all of your Applications (and other purchased content) onto that external storage. Drag all that to where it goes in to the new iTunes as well. Pictures, if you have any, can be dragged out of iPhoto or the Pictures folder, and onto external storage.
Itunes Reinstall Windows
From there just drop them into My Pictures or wherever you'd like on the new computer. When you connect your iPhone to the new computer, all of your things will be there, somewhere, and you just have to point your iPhone to them. It will wipe your iPhone, inevitably, as you go to a new computer with a new iTunes Library. The only work left is to go through and check what you do and don't want synced to the iPhone when it's connected.