Been through this when I was thinking of selling mine and the short answer is, unfortunately, there isn't anything that anyone outside of a repair centre can do. Apparently Canon used to offer it as a service when the camera was with them but they've stopped doing that.
The usual suspects - MyShutterCount and EOSCount don't work with the original 5D - I've tried them and the camera simply doesn't do what they need to give a shutter count.
The only thing that might help is if you've used the continuous file numbering from new and not reset it - in that case the file number should be representative of the shutter actuations.