rockyjohn :
Your problem may be the proprietary Dell BIOS that could affect a lot of things. First stop might be the Dell Support site to find out all you can about what the BIOS might let you do and not do, probably not an easy task unless you want to simplify by limiting replacements to what they offer as replacements - not necessarily buying from them but buying only those items the list.
The CPU you might be your biggest issue - I can't imagine it not accepting any reasonable HD. Also not sure about OS - what are you doing about that?
I suggest thoroughly researching the Dell forum - hopefully they have posts from others trying the same thing.
Sorry for the long delay in replying. After getting some info that seemed to point to the Q9400 not being a "usable" processor on the Dell board, I swapped it down to a E8400 Duo core. Installed it and I still had the same processor error. Next, I repeatedly tried rebooting and finally was able to boot from the Vista backup disk I have for my XPS notebook. Once I did that, I found a new bios rev A07 vs A04 and flashed the bios.
So that took care of the processor issue, my guess is it would have made the Q9400 usable also. Oh well.
The next issue still exists. My Plextor SATA DVD rom doesn't seem to respond to commands. It has power and functions as long as the media has some sort of boot command from Dell/Vista. I CANNOT get it to spin up a video card install disk or the wireless card disk?? I've updated the firmware/drivers, checked the bios config etc and nothing. It's up and running now, but somewhat disabled. Had to put it down for a few days.
The OS I installed was from the restore disk for my notebook, it's the same OS that the 420 originally came with. Would that be of any issue since I installed it over a HDD that had XP without doing a full install from a real Vista disk?