On your existing computer, BEFORE YOU DO ANTHING, bring up your desktop; go to Start - Run - (type in) regedit, and do the following:
Hit the + signs on HKeyLocalMachine - Software - Microsoft - Windows.
Single left click Current Version and write down the Product ID which you will see in the right panel.
Call M$ tech support and tell them your story and give them this ID.
It's all that I have to share with you. Maybe it will be enough to convince them to help you.