Playing Vedeo's on You Tube is probably one of the most stressfull things you could get that computer to do.  It loads up the Internet Connection, the Video card and the mainboard Ram & CPU.  If there is a problem with the Power Supply or the Video card this is when it will become evident.

My money is still on either the Power Supply, the Video card (Not the Drivers as you have now replaced those with he latest version) but most likely it is one or more bulging Capacitors on the Mainboard.

Have you been able to get a "blue screen of death" screen out of it yet, see instructions in my earlier post.

You could also try:
Check that the CPU and Power Supply fans are clean and spinning at the correct speed, slow or excessively dirty fans can cause the CPU or power supply to run hot and this can cause intermittent rebooting.  Try blowing a desk fan through the system case in the same direction as the normal air flow and see if that makes any difference.  You can also use a hair dryer but make sure it is on the COLD setting.  You could also check the BIOS settings to see what temp the CPU is running at.

I would also remove the external Hard Drive if for nothing else to remove it from any.

If you go to this Dell website link http://www.dell.com/support/troubleshooting/us/en/19/ProductSelector and enter the service code ID as "971LR1S"  yiu will be able to view the original specification details of your System when it left the Dell Factory.  It may have subsequently been changed.  It should be a "Dell Inspiron 530".

...put it in the oven then.

I would be interested to hear how dusty it was inside, when you get to it. I thought of that a day or so ago, but not all of what you describe could be attributable to dust

I have the BSOD sitting there now!

No name for the stop error the numbers are
0x0000008e (0xc0000005, 0xbf81c022, 0xb820796c, 0x00000000)

Message does say to check for bios updates, pretty sure I got the latest off Dell last week (but BSOD will go if I check what version)

further message details

win32k.sys - address BF81c022 base at BF800000, date stamp 4f85831a

I can leave the BSOD there for a while in case you need anything else

service tag # for the machine is 971LR1S

You might be on to it there.  ". . . the scan on Dell's site only picked up 2GB, saying 2 slots were empty. It looks like both 512's aren't working."

Read the article at this link http://support.microsoft.com/kb/315335 but disregard the references to Setup as I have also experineced the Stop 0x50 error also mentioned here many times and every time without fail it has been faulty Ram or on some occassions dirty Ram contacts.  It may also be a fault in the bus under the ram slot on the Motherboard itself which unfortunately is unfixable so either don't use that slot or replace the mainboard.

Start by removing the 2 sticks of Ram that the Dell Ram check did not recognise, run the machine as you did when it crashed and see what happens.  You may need to remove all the Ram & try individually or in combinations and in different slots to isolate which is the faulty module or slot.

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Very important:  never remove or insert Ram Modules while the Computer is going ald also, Remember that when handling ram you need to take proper and adequate Static precautions and do not touch the chips on the modules themselves, alway handle them by gripping on the edges at each end.
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You could also try cleaning the contacts on the bottom of the Ram Modules with a clean Cotton Bud and some Isopropol Alchol (Can get from you local friendly chemist if you don't have any but you may be asked for ID as it is used in P manufacture so don't ask for too much).  Note:  You can keep what is left in the fridge for later use.  Also try cleaning out the Ram slots themselves by firstly sucking or blowing out any loose dust surrounding them and then cleaning gently with said same Cotton Bud an Alchol as detailed above.  Be gentlle and patient, make sure that you do not leave any "fluff" from the cotton bud on the pins inside the slot but also make sure you apply sufficient pressue to clean the pins.

Hope this helps.

I think I will just retire it, maybe do a rebuild after all it isn't essential to me.

Got a "pagefault in non paged area" error message yesterday.

Have spent the last few days trying to get all the info I neede off it, think I have now finished.
Started off by just copying files onto external drive but even then it would keep crashing - and the external drive is suspect anyway. In the end I just made all files accessible and sucked them off from the wxpc