Yuck, that's no good. Provided you have the data, you're golden...
Under Windows, all the data is stored in this path:
Provided the username is the same from the old computer to the new, there's nothing you need to do except copy that data over.
If the folder paths are different, you'll need to make some edits to the rekordbox3.settings file. Crack it open with a text editor and change the path anywhere your old username appears, swap it with your new one.
Let me know if you need further assistance with that.