In order to get to the registry, when the above listed error appears, hit Shift-F10. This will bring up a command prompt window. Type regedit and then make the registry change. At that point, reboot the machine and it should boot up into Windows 7.
NOTE: This was performed on a Windows 7 64-bit machine. Per a reply to this blog, it was also confirmed to work on Windows Server 2008.
Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SYSTEM\Setup\Status\ChildCompletion] "setup.exe"=dword:00000003