Until I open it up again and have a look I'm not sure. I know it's physically pretty large by modern standards, nearly A5 size and, IIRC, no cooling fans.

It's a Compaq Pressario S000 DF131A, the graphics card slot is moderately short - less than the length of the card - and is seated in a brown socket that finishes an inch or so from the card mounting bracket. I think it's an AGP socket.

There's a lot of dust in the PC so a clean is called for.

I won't have chance to do any more with it until tomorrow afternoon at the earliest.

I agree with the PSU being suspect, had it happen myself in the past. Lights on mobo & case but PC wouldn't boot, wasn't giving the usual "boot" sound after POST, instead the PC shut down. Mine gave a crack sound when the PSU failed and although there were still case lights and power to the mobo etc there wasn't enough power to boot. For the times you can boot there than might not be enough power for the graphics card.

Another past experience was a HDD fault where the disk was still spinning but not fast enough to boot, it would boot occasionally but not reliably. I managed to clone the HDD to a replacement which then booted OK.

Try booting off a Linux Live CD too, the disk checker might flag up a disk fault almost straight away. I've done Windows scans and disks given the all clear but Linux flags a disk error...... more confusion.

I did all the fix work (fixmbr, fixpartition) with a perfectly working PC, everything, including the graphics card just fine.

It's only today's inability to display anything that makes me suspect the graphics card now. But given the amount of dust inside it could well be that returning the PC, and its short car journey, may have affected the card/MB interface. Perhaps the journey back here has reset it again .

It also has two unneeded cards including a wireless lan card which I will probably remove, that will reduce PSU consumption fractionally.

Look for bulging capacitors on the motherboard, a nice free thing to check.

It might also be a good time to strip the PC down totally and put back together, to cure any issues of that sort.

There's a manual for that computer here, which suggests that it has on-board graphics. So you should be able to use that rather than trying to find a new card.

With the varying symptoms that you have experienced I would have to agree with others that a PSU problem is a possibility. That can lead to all sorts of strange, inconsistent behaviour. Problem is a PSU for something that old is going to be difficult to find.

I'd seen that manual. The relevant (I think) bit is

An expansion card is a circuit board that fits into a PC expansion slot. Your PC contains several expansion slots that can be used to add components to your PC.

Component configurations vary by model.

Expansion cards you can replace or add to your PC include:
...
� Graphics Card
...

I read that as meaning a graphics card could already be there, and replaced, or a graphics card could be added.

Although there is an Nvidia chip on the motherboard the only VGA connector is on the graphics card itself. If there had been another I'd have tried that.

I'll remove the wireless LAN card and the other card (may be a modem) to marginally reduce the PSU load. The 2 CD drives work fine.