-Start with bigger new drive, 1TB.
-Make a small partition on it, say 100GB. Set the partition as a SYSTEM, Bootable type partition when creating it.
-Next, using a tool that can create a true image (exact copy) of the original hard drive you duplicate the existing 80 gig onto the 100 MB partition. This is called imaging the drive. WD and Seagate have utilities for doing this. They will only work if both drives are the same brand WD or Seagate. Acronis and possibly others have drive imaging software.
-Remove the old system drive (80 GB one) and you should now boot off the 1TB drive with 40 GB free space.
-Use the rest of the TB for audio, video, bulky programs like games, etc.
Avoid rebooting with both bootable drives in the PC. SOmetimes Windows XP gets nasty when you do this. It was a copy proteciton scheme issue for a while.