If you want a graphical tool to find the DN of an Active Directory object then the free Microsoft tool LDP.exe should do the trick. The tool is included with the Windows Server OS and can be accessed from your local computer if you have the Windows Server 2003 Admin Pack or the 2008 RSAT installed.

Note: The steps below are from the 2008 R2 RSAT version of LDP, the process is very similar for the 2003 version however some of the memu names etc at a bit different.