Just unplug the HD from one PC and plug it into the other, power cable and the IDE or SATA cable. Set the jumpers on the back of the drive to master if it is a ATA drive, the settings can be found on the drive's maker's website. Set the boot sequence in the new PC or check to make sure that HDD0 if it is master is on the boot list and remove any bootable media. I think you might have to reinstall because of a different harware configuration.