I have been given a laptop, which does absolutely nothing at all - no lights on it, no fans, nothing.
The delta power adaptor has a green light on it.
As soon as it is connected to the laptop, it goes pale and starts blinking.
The power supply gives a steady 19.4V as per spec.
The laptop is an old packard bell of unknown origin, but it's running windows XP (from the sticker underneath) so it can't be too old.
Anyone have any ideas? I'm thinking a motherboard short?

Take out the battery and disconnect the power supply. Let the laptop sit for a couple of minutes. Attach just the power supply and try to start the system. If this doesn't work, disconnect the power supply, let the system sit for a couple more minutes, insert the battery, and try again. If neither of these steps work, look for a reset switch somewhere on the machine. They usually are little pinholes on the bottom of the machine. If you find one, depress it for about ten seconds and try the above steps again.

If this doesn't work, you may have gotten a bad laptop. Did the person who gave it to you indicate that it worked or not?

it's now started sparking when you insert the plug...
The guy said that it was working, but recently he tried it and nothing happened, so he sent it round to me.

Sparking is bad. Sparking means that the component is malfunctioning. What sparked? Whatever it is, stop using it and get it replaced immediately or you may damage other components.

Well when I plugged the power adaptor plug into the laptop motherboard, it came from that join.
Power adaptor appears to be giving out the proper voltage on a multimeter.
So is that the motherboard failing?
is the little card running sideways from the processor but above the main board part of motherboard??

Did you try what I originally suggested? Did it have any effect?

The power connector is typically located on the motherboard PCB. Do you think that the plug is the problem and not some other hardware failure?

it was after I tried the original suggestion that it started sparking visibly.
I couldn't find a reset switch on it though, but I'll look again.
Since the psu appears to be giving a normal output that probably means it's a motherboard problem?

I've pulled the whole thing to pieces and the power board was separate to the motherboard. The sparks only occur when the power board is connected to the motherboard, so either the motherboard is dead or the power board only faults when the motherboard is present.
I've been quoted £200 for a new motherboard, and I can't find any on ebay, so it looks like this is about to go in the bin...

Are you sure the power adapter polarity is correct for the laptop. Just because the voltage is correct doesn't mean the adapter will work. You need to make sure the amperage and the polarity of the power adapter are correct as well. Another problem could be a damaged DC power jack on the motherboard, or power board in this case. If the jack solder joints crack and the jack is loose it can cause sparking. If the jack appears damaged (center pin bent or plastic insulation cracked) then it should be replaced. Even if the estimated repair cost is more than you are willing to pay, don't dump the laptop in the trash bin. I'm sure there are many parts that could be salvaged from the laptop that someone else could use to repair their own laptop. I never throw a laptop away until I have salvaged every part that could possibly be used. Even then, try to make sure it goes to a recycler.

Are you sure the power adapter polarity is correct for the laptop.

Well it's the same power adaptor as has always been used with it, so it should be fine shouldn't it?

The power socket is in pristine condition.

I did look at selling the parts on ebay but they're mostly worth about £5 by the looks of it.

If it's the original adapter that came with the laptop it should be correct. It soulds like there is a short in either the power board or the motherboard. What is the brand and model is the laptop?

It's a packard bell iGo 2000/4000
I do have all the serial numbers if they're any use to you.

The laptop looks similar to an Compaq Presario 700 but not exactly like it. I also found some information that the laptop may be an NEC clone. If you don't feel the laptop is worth repairing, maybe you could sell it for parts. I'm sure someone could use it. I usually get any newer parts laptops I can find.

I might put it up on ebay - there seems to be one or two on there now and again.

The fact that the power supply LED is flickering out when you connect it to the laptop, and the spark you're seeing from the connector, both indicate that there's a short circuit on your motherboard. It's causing the power supply's overload protection circuit to trigger.

You should disconnect everything possible from the laptop's motherboard, to see if the short is in one of the peripherals. Does it still short when it's just the power regulator and the bare motherboard?

If not, one of your peripherals is causing the short, and you should reconnect them one at a time until it fails to determine which component it is.

If so, there's a short on the motherboard itself. You should look for visibly swelling capacitors or burnt power regulator ICs, probably close to where the power regulator board connects.