I upgraded to Exchange 2007 around 6 years ago and thought I had completely removed Exchange 2003 from my infrastructure.
Now I'm trying to install Exchange 2013 and the install is saying that I still have a 2007 server.
Is there a recommended way on resolving this problem without having to find my Exchange 2003 installer/
Microsoft have some good resources on this. See here
IF you have already done this, then you may need to use adsiedit to clean up legacy attributes. Before attempting this, I would recommend you do some research on removing exchange 2003 via adsiedit, there are some good blogs around that cover this. Example