Boot problems

What do you mean by "reformatted"? Do you mean you ran a Format operation on the disk while it was installed on the "other PC" and then left it empty before re-installing in the Dell? Then of course it cannot load any OS from the hard drive.

Or, do you mean that you installed XP on the reformatted disk while it was still installed in your "other PC", then re-installed the disk on the Dell? In that case the installation probably put some wrong drivers in, because the Install process detects all the devices (mobo hardware as well as added peripherals) in the machine it's running in and installs them. When you later moved it to your Dell, it does not have drivers you need to access lots of things and it cannot read the hard drive.

To fix this, the ideal situation would be to get the Windows Install disk you used and do a completely new Install while the drive is in the Dell machine. In starting that, be sure to tell it to delete any existing Partitions and start with a blank hard drive.

If you cannot do the Install operation in your Dell, the next-best option still requires the Install Disk you used. But this time when you start it up, you choose to do a Repair Install. That will re-do the inventory of devices needing drivers, then remove the wrong ones and add the ones that are missing. When done you cancel out of the Install, remove the disk from the CD drive and reboot. That should get it all working.