No, there's a jumper on the motherboard you switch over for 30 seconds - 1 minute while the computer is shutdown and disconnected from the wall outlet. You can also pop out the battery on the motherboard for a minute or so.
Originally Posted by xxjon
and the OS was orginaly installed on second computer, just put into first to see if it would work,
There's your problem.
OK, you'll either need to do a full reinstall of Windows while the drive is in the first computer, or do a repair install as shown here: XP Repair install
. You can't just swap hard drives from one computer to another and expect the OS to start right up. The motherboards use different drivers and addresses for their components and Windoes will just refuse to operate when it can't resolve those issues.