I had a similar problem with my previous test box - basically, the 5.6 install routines were not recognizing my CD-ROM drive (an NEC SCSI drive that works fine on 5.1.2). The conclusion I came to is that RH 7.3 no longer supports older hardware (in my case, the box is a Dell PowerEdge SP5133 server - one of Dell's earliest servers). I acquired a newer box (a PPro 180 with IDE CD drive) which 5.6 installed on clean as a whistle.
Des Dougan