When your authorise your computer for your iTunes account iTunes stores some encrypted data on the hard drive to register it, and makes a note back at base about how many computers are currently authorized. If you wipe the hard drive to reinstall Windows you erase that marker, but not the record at HQ. Since you are only allowed five authorized computers at any one time losing track of one of those limited authorizations is to be avoided.
You iTunes library is the the iTunes folder containing the library database files and all of the media files that it is connected to. Typically these are all inside the iTunes folder but it may be that you have media scattered elsewhere. If you've implemented the advice in the user tip you should have picked up on any satellite locations. The user tip is concerned with your iTunes library, but any other documents, media or data that is on your computer is likely to be stored on the system drive. This should all be backed up too, ideally to two separate drives if you can. It is much easier to backup more than you need and discard it later than not backup enough and mourn the loss when it is too late to recover something vital.
You should really back up the entire C:\Users folder from the current system. Then you can selectively restore what you want. The main benefit of using SyncToy over simply copying and pasting with Windows Explorer is that having set the system up you can run it again and again, each time reviewing and then updating only the changes to the source. Then, should your main drive fail, you have a backup that is up to date. I have set this up for some people with separate folder pairs for the main sections of the profile, e.g. Desktop, Documents, Downloads, etc. Then these can be updated independently or as a group.
If you restore the backup of the iTunes folder into the Music folder of your new profile before installing iTunes you won't have to contend with an empty, but newer, set of data files to overwrite.
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