XP doesn't boot: black screen, blinking underscore, lots of swearing

Hello everyone!
I have this issue on an old pc (Athlon on MSI KT3 Ultra 2) here are the facts:
As the BIOS finds the HD, a black screen with blinking cursor in the top left corner shows up and nothing happens.
I tried to boot from the Windows XP original CD, but after the first checks on the HW it goes on the same black screen right before loading the screen in which you choose whether to install or restore windows.
I thought I had some HW problems so I used several bootable diagnostic utilities for HD and RAM but no errors nor warnings were detected.
I removed the HD and connected it into a newer PC and the boot from CD went fine, so I've restored Windows and after that this PC correctly booted the OS; I thought I fixed the problem so I connected back the HD into its old PC and....black screen again!
I've replaced the IDE flat, tried all the possible combinations of RAM sticks, reset the BIOS through its jumper...nothing!
I then tried to connect the newer PC HD (which runs XP too) into the old PC and guess what: everything works just fine! I guess none of the parts is physically faulty so it MUST be a SW problem.
I think it might be a video drivers issue because it freezes right when the video card (Radeon 7000) should kick in, but I don't know how to solve that since:
-the old PC doesn't boot the safe mode whatsoever, not by pressing F8 nor by ticking "safe boot" in the msconfig console while using the newer PC
-i can't delete tose ATI drivers while using the newer pc because it says there's something wrong with a dll
-I can't overwrite those ATI drivers while using the ATI CD on the newer pc because, since the machine doesn't find the card physically connected i guess, it aborts the install sequence.
basically: I can't figure out a way to force the old pc into running the basic VGA!
Does anybody kindly have a good tip for me?
 
boot in safe mode if you can

go to device manager and uninstall your video drivers, reboot

hope this helps

another thing check your psu maybe it's not giving enough juice :)
 
Howdy!
There' no way to boot the safe mode :(
Thank you for you advice, but curiously the PSU is a 400W brand new (pretty exaggerated for such an old pc!).
I got rid of all those ATI drivers and utilities but it didn't work either; besides, I tried using a different video card and it didn't solve it, so I swept this cause off the list...
 
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