At the risk of causing some anger,
I'm sorry to say, but all such questions on our site get the same answer :
We cannot diagnose hardware problems from far away.
You have changed the RAM, but as you give no details about the computer and
the new RAM, we don't even know if it fits your computer.
If you have run memtest while the computer was still booting and shown us
the results, we could have analyzed whether the errors came from the RAM
or from the motherboard.
You had errors with the disk and replaced it, but we don't know what exactly
were the errors before it was replaced.
You have done the "fixing on a series of issues", but what did you do?
You are currently spending money on trying to revive a 9-years old computer.
For your information,
not many computers survive to that age, and I think that yours didn't either.
You may take your computer to a professional repairman, who will have the tools
to pinpoint the problem or problems. But this is again money spent, and the
diagnosis might probably be that the repair is too costly to be justified.
So let me give you my best advice : Return to the shop every new part that
you have bought, if you can still be reimbursed.
Then invest your money in a new computer.
Believe me, your computer is not worth it, and you might find yourself
spending on it more money than the price of a new one.
And what will you have after fixing it ?
A 9-year old computer which is vastly inferior to even a cheap new computer.
Technology has much advanced in the last 9 years.
Excuse me for this rant, but I have seen too many such cases pass here.