I do have a Project 2003 file called SomeProject.mpp in the active directory, so I don't know why it couldn't be opened.
I don't have access to any version of Project at the moment, but do recall that Microsoft changed some details between Project 98 and Project 2000. They might well have done the same for 2003. ActiveState provides an OLE Browser (it's in the start menu somewhere). I'd look first to see if the name of the OLE Control for dealing with Project has changed, or if it expects different parameters.