Comparing REG Files using WinDiff (check your inbox)
In Registry Editor, locate and click the registry key or subkey that you want to back up > Select File > Export.
In the Export Registry File dialog box, select the location to which you want to save the backup copy.
Open WinDiff. On the File menu, click Compare Files.
Type the path and name of the first file, and click Open, repeat the same with second file.
In the View menu, click Expand, or double-click the files in the list.
WinDiff combines both files and highlights the differences in red and yellow.
Lines present in the first file but not in the second, are red.
Lines present in the second file but not in the first, are yellow.
Lines, present in both files, are white.
Also, Regshot is an open-source (LGPL) registry compare utility that allows you to quickly take a snapshot of your registry and then compare it with a second one.