Your Home folder is the folder that has your user name, and contains all the "My Documents", "My Pictures", etc. folders. The registration file has a .REG extension and is named like the product.
However, it's not really a problem if you lose it, there's nothing special about the .REG file, it's just a standard registry export file, so you can double-click on it to insert the Serial number in the registry, its only purpose it to save you from typing it manually when reinstalling.
