Check to see whether the path for "Default" under HKEY_CLASSES_ROOT\MSCFile\Shell\Open\Command is correct. This value should be a REG_EXPAND_SZ (not REG_SZ) and should have the value: %SystemRoot%\system32\mmc.exe "%1" %*
So, I fired up my Regedit and indeed, the type of the default value of the key in question was REG_SZ. But how do I change it to REG_EXPAND_SZ? Fiddled with Regedit for a while to no avail. You can't delete the default value to recreate it, and there's no option to change the value's type. So I ended up writing a small program in Delphi to do it. The program (with its trivial source in Delphi) is here. Just click the "Fix it" button.