Some SLP OEM disks will just run like a normal off the shelf disk if used on a different brand PC. If it can't find the matching BIOS code it just prompts for you to enter a product code like a normal disk would. Some won't work at all if it isn't that exact make and model PC. Even if you use a Dell Windows 7 disk on a Dell PC for SLP to work it has to have a Windows 7 SLIC table. If that PC only ever came from the factory with Windows Vista or XP on it, the BIOS would have to be updated for Windows 7 SLP activation to work. Its that way on purpose to stop free upgrades to Windows 7 via bit-torrent.