Can you confirm what you have installed in terms of hard-drive and cd/dvdrom in the problem PC? Also if each is on its own ribbon connector; or are they both on the same ribbon?
Have you ensured that jumpers on both HD and the dvd/cdrom(or burner) are correctly set?
Depending on what is installed physically and how... depends how the jumpers are set... If they are not set correctly... you can get the boot error message problems you have...?
Also what was happening when this problem first appeared; what was going on then; what were you doing or atttemtpting to do at the time??
In answer to your most recent query here; can you transer the HD from one PC to another and safeguard data?
You can connect the HD in the problem PC to another system in at least two ways.
Either remove the HD and use a EIDE/SATA-usb adapter that allows you to connect the HD directly - via an adapater-plug/socket - to a usb input on another working PC. Then you can access all data etc and transfer it to dvd/cd media. These adapters cost onl a few $$$/£££'s; come with adequate instrucitons - and are self powered (they have their own psu).
Another way is to install that HD in another working PC - as a Slave to the Master. Or and better/safer, as a second HD - on its own ribbon/controller in that working PC. And then again access it contents and copy to dvd/cd media...
When you downloaded the linux variant (ubuntu in your case?)... that was an ISO file/image. That you have to burn to a dvd; and then boot with that dvd. You cannot simply copy the ISO to a cd/dvd and hope it will boot; 'cos it won't... You can also download knoppix (another linux variant) and again burn the ISO (image) to a dvd and boot with that...
Do not throw the kit away... hang in there and one way or another... hopefully...?