Sorry it's late but have been experiencing exactly the same problem on one of my PC's the last few days also.
"Although not resolved the problem yet"
First of all it started out being a faulty graphics card that caused my original problem (it blew up) although it did not fail immediately.
However even after changing the card, formatting the HDD and reinstalling XP, it still would not run properly, although it did seem to settle down "for a short while" (couple of hours)
At the moment I am now suspecting either a faulty power supply (which is going to be replaced tomorrow) or possibly even the main board (which has had a slight problem for the last few years)
But most of the time can't get past the BSOD or it freezes on either the Windows boot screen or Windows startup screen.
Regardless of any of the
STOP ERROR messages, windows sometime does boot-up after several hard boots.
I don't know if any of the above info is of any help with your problem, but initially it sounds like you have a graphics card issue.
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edit 1
Just a update.
Although I changed the PSU (Power Supply Unit) it's not really made that much difference to the way it "sometimes" starts up. (So probably not that)
However, I'm just running a full HDD test with the setting to "Automatically Repair" any bad sectors etc.
(Although you need to get into your PC to start the option,,,, unless any one can advise you on how to run CHKDSK from a DOS prompt or disc)
So far it has found at least one error.
(Windows replaced bad clusters in file 3009 of name \windows\system32\config\secEvent.Evt.)
This error probably occurred from one of the many "hard boots" the PC has suffered over the last few days since the original problem.
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edit 2
Not wishing to crow about it, but I seem to have solved my problem.
(Hope the OP is getting on just as well)
My problem seemed to start from when the Graphics card went faulty.
Even after installing a new card the system still was not as good as it used to be and still froze quite frequently.
To cut a long story short.
What seemed to have happened was, when the system originally went belly-up it damaged one of the sectors on my HDD (although it was not the OS HDD)
Even after CHKDSK repaired the bad segment it made no difference.
But if you recall I also said earlier that I had also re-installed XP so as to have a nice fresh install with the new graphics card.
Although after the install I had also lost approx 2GB of space compared with what I normally use (and had not even finished loading all of my programs)
The problem was now 2 fold, possibly a faulty re-install, plus re-installing onto a PC with a faulty HDD, that was due to the earlier crash.
But before I actually decided to throw the damn thing out of the window, as I had now spent over a week (10+ hours a day)
I first decided to re-ghost it back from when it "was" actually running properly, (nothing ventured, nothing lost) and yes, you probably guessed it, "IT NOW WORKS FINE"
Conclusion:
Faulty graphics card caused a crash which damaged a HDD.
Faulty XP re-install due to damaged HDD.
Hope you have as much good luck as I have and any of the above "Saga" comes in handy.
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If it ain't broke.
Don't fix it.