XP disk validation

Hello,

I purchase years ago a n XP pro at Staples.
I used it on a computer. I trashed the old computer and I am trying to install Xp pro to a new computer.

I lost the key and I found a video showing that the key is in the disk at:
x:\I396\unattend.txt. I fouund the key number, but when I tried to use it. the installation did not accepted.

Please advise

Thank you

Tuli