You don't say whether your HHD is IDE or SATA. I'm assuming that it is IDE and you may therefore need to ensure that your jumpers are set correctly before your new PC will see it. This is particularly important if your new PC has a SATA drive and your old drive is IDE. If you set up the old drive as primary, your PC may look for the OS on this drive rather than going directly to the SATA drive. You need to check that your motherboard will support both types of HHD. You may find this helpful
If your mobo does not suport both types, you may need to get a USB HHD holder. This will also be useful if you subsequently wnat to use your old IDE drive as off-line back-up/storage.