Even when dealing with Windows 2003 you can still run into weird errors.
I had one of those this morning. Installed a server with an unattendend install. After installation I tried to start up the Local Security Policy MMC and found this staring me into the face:
MMC Cannot open the file C:\windows\system32\secpol.msc
This may be because the file does not exist, is not an MMC concolse, ro was created by a later version of MMC. This may also be because you do not have sufficient access rights to the file.
Of course I checked the file and the rights and they were perfectly normal. The file was not damaged either, it worked fine on another machine.
Googling a bit didn't reveal much. Other people either could not run any MSC file at all or they had weird errors with admin tools on XP-64 bit. Going trough google groups did get me thinking in the right direction. Turned out all I needed to do was to go to:
C:\Documents and Settings\
And delete the secpol file there.