Regmon is a Registry monitoring utility that will show you which applications are accessing your Registry, which keys they are accessing, and the Registry data that they are reading and writing - all in real-time. This advanced utility takes you one step beyond what static Registry tools can do, to let you see and understand exactly how programs use the Registry. With static tools you might be able to see what Registry values and keys changed. With Regmon you'll see how the values and keys changed..
Regmon works on Windows NT/2000/XP, .NET Server 2003 (RC2 and higher), Windows 95/98/Me and Windows 64-bit/Itanium.

http://www.sysinternals.com/ntw2k/source/regmon.shtml