You can try making a backup image of the 80gb XP drive, then reimaging it to 80gb+ unallocated space you create on the Gateway's 1tb HD. I would use free Macrium Reflect
which will make you a Linux boot disk to do the reimage.
When you move XP to different hardware it requires a Repair Install before you even try to start it up. This is easy with XP because the Repair Install is done from the booted CD. It is not the first "R" given for Repair (that's a Recovery console) but proceed as if to install until it locates the installation and then choose "R" for Repair if offered. If not offered, it is not reparable and will not start on the new hardware.
Once you run a successful Repair Install it will take over the bootloader from Windows 7. You then need to add Windows 7 to Dual Boot using EasyBCD 2.0 beta
, which will require Net Framework 2.0
to be installed to run in XP. Just add Windows 7 by type and correct drive letter on the Add/Remove tab.
If this fails, check back on how to restart Windows 7 so that you can add XP from Windows 7 which works better. Add the 250gb HD (giving it a letter in Disk mgmt) after you complete all steps.
Your backup plan is to clean install XP to the new partition you create using this tutorial: Dual Boot Installation with Windows 7 and XP