You backup what you can't lose, get out your W7 DVD and start over. If the CD Key fails you call it in. Microsoft has been really great at resolving legitimate license failures.
But there were so many cracked W7 installs where the shop used a crack copy and later, a story is told like yours.
Here I assume you have a legitimate DVD and key.
My son has a computer that was custom built specifically for gaming. Windows 7 was installed and worked fine for 2 years. He received the message and began researching how to correct the issue. After multiple attempts with no luck, he came upon a "solution" that required him to delete a file on the operating system to remove the message. Now that the file has been removed, he can no longer enter a product code to fix his problem. How can we find out what file this was and correct the problem?