On the server you have MOSS 2007 installed, copy PRESCAN.EXE and
PREUPGRADESCANCONFIG.XML from C:\Program Files\Common Files\Microsoft
Shared\web server extensions\12\BIN to some folder on your SPS 2003
server. I've done that successfully several times now.
Also, a bit of advice: If you get errors on the prescan, you will more
than likely have to get the hotfix that is described in
http://support.microsoft.com/kb/918744.