Memory Stick BackUp

How exactly did you backup your iTunes, did you back up the whole iTunes folder (including the iTunes Music folder and the iTunes library file or did you just back up the media files (songs etc.)?

If you backed up the whole folder then simply copy it back to your default Music folder in Windows. Once it's been copied over open iTunes while holding down the Shift key. You will get a box opening giving you the option to create a new library or navigate to an existing one. Navigate to where you copied the iTunes folder and choose the library file that's there: How to open an alternate iTunes Library file

If you only backed up the media files then you need to add them to your new iTunes library. Open iTunes, go it Edit>Preferences>Advanced and check the box "Copy files to iTunes Music Folder when adding to library". Connect your USB drive and either use the option File>Add folder to library and navigate to the drive or open the drive in Windows Explorer select all the media files on it and drag them to your iTunes source list: Adding Content to iTunes