Can't install Quicktime 6.5.2 or FCP 4

It's long-winded, but please bare with me. I have just erased my Macintosh HD drive so I could start fresh. It rebooted fine and I upgraded to OS X 10.3.9. I had Quicktime 4 and needed to upgrade to Quicktime 6 so I could register it for Quicktime Pro for my Pro applications. So I installed QT 6.5.2. After I did that I installed my Final Cut Pro HD and then tried to register for QT Pro. It said I didn't have the right serial number. So then I installed the QT 6.4 reinstaller. Again the serial number didn't match. At that point I can't even open FCP because it says it needs QT 6.5 or higher. So I try to upgrade QT back to 6.5. The installer opens fine and my Mac HD volume has the green arrow, but once it starts it quits. The whole window disappears and there's no error message or anything. I've re-downloaded it a few times and it does the same thing. I repaired the disc and repaired disk permissions, shut down and rebooted - the same problem. Now I'm stuck with QT 6.4 and can't open my FCP! Any help would be greatly appreciated.