If you're trying to reinstall the operating system on the original OEM PC, the installation key should be listed on a COA (Certificate of Authentification) sticker affixed to the PC tower somewhere. If you don't have the original OEM PC, there's a good chance the operating system will not install properly because installation disks that come from a large PC manufacturer are branded. That means during the installation, the OS will check the PC's BIOS for the original manufacturers branding information. If the OS doesn't locate the branding info, the installation process will most likely stop.
If you want to find out what the OEM installation key should be for your copy of the Operating System, do the following:
1) Place the installation disk into the PC's CD/DVD tray.
2) Open "Windows Explorer" and Click on "My Computer".
3) Click on your CD/DVD drive to explore the contents of the installation disk.
4) Click to open the "I386" folder, then locate the file named "unattended.txt".
5) Open that file with your text editor (notepad).
6) At, or near, the bottom of the text file, you should find a line named "ProductKey".
The installation key for that OEM copy of the OS will be listed in that line. Even knowing that key may do you no good IF the PC you're trying to install the OS into is not an OEM PC from the company the branding is for.