It seems like Office 2003 and 2007 don't allow to have any rest of former
installations present when installing.
I - and a lot of others have had the very same problem, and mine were solved
by the way, I explained in the answer to "setup installation files runs" the
01-14-2008.
Why you want to run OL2003 instead of OL2007 I don't know, but I don't think
you can - Outlook can't be discharged when installing as far as I know.
There was another solution to this problem, but explained in the Windows
Vista administation fora about the 7.th or 8. of january- but that I did try,
and it did not work for me.