Importing Registry settings from an existing file can be as simple as double-clicking a REG file, or using the Import feature from the File (or Registry) menu in Registry Editor. While importing settings isn’t difficult, it’s important to understand the implications of importing different types of files.
When you import a REG file, any settings (keys, values, and data) that are not currently found in the Registry are added. Where data settings in the file are different from existing settings, those settings will be changed to reflect the contents of the file being imported. This is an important consideration. For example, let’s say that you wanted to remove a set of values and data that you had added manually using Registry Editor – double-clicking on the REG file that you created before making the changes would not remove those settings. When a REG file is imported, it simply merges new data into the Registry, and changes existing values as required.
This is one of the reasons why exporting the Registry as a hive file (in XP) is a great idea.
Although you cannot manually edit these files, they create a binary “snapshot” of all existing Registry settings. Then, when you choose to import a hive file, all existing settings in that particular key are completely overwritten with the contents of the hive file. For example, let’s say that you were to export to a hive file, add entries to that same key using Registry Editor, and then import the hive file – the whole key would be overwritten with the settings in the hive file, along with those manual additions. Importing hive files is a quick and easy way to “get back where you were” in cases your manual edits go awry.