If you have tried the HDD and RAM then it does sound like the motherboard is dead, it may be fixable but it is probably the power IC or the caps have given up, they are usually solid state and not as handy to pick up in an electronics store.

You could buy a motherboard to replace it but the options are not very good:

It will be at least 2nd hand

You won't know it's history

You don't know how long it will last

And you will have little recourse if anything goes wrong

I know getting used to a new system is a big deal but other than visuals and security Windows 7/8/10 is basically XP. They all serve the same purpose.

So my advice would be to go get yourself a new laptop , you can pick up a nice system for a good price that suits you. The best part is that all the things you put up with on your old system will be gone, you will literally be swapping it for a younger model. That is the best advice I can give.