The 80 has at least 1 partition as do all drives that are formatted and contain data. To determine whether it's Primary or Extended partition right click My Computer, select Manage, and select Disk Management which should show a Drive0 and a Drive! which will be the 20GB and 80GB drives respectively for your setup. The 20GB will have a bar of 1 color in its window which is the color for a Primary partition, and the 80GB drive will have a bar in its window either the same color if Primary partition or a different color if Extended partition. For my system Primary partition has dark blue bar while Extended partition has medium blue bar.
In any event you probably should backup your data to external hard drive, DVD before beginning repartition and reformat or other backup media; you can also save your image files there. Be sure to do an integrity check of image or verify image after backup images are created and before repartitioning the 80GB drive. Ghost calls it an Integrity Check while Seagate Disc Wizard uses the term Verify Image.
Next step is to repartition the 80GB drive which can be done by booting to WinXP install CD and run setup, or the Maxblast or Seagate Disc Wizard can be used. If using WinXP CD, you'll select the "Remove Partition" option which will remove the partition which will thereby lose all data on the drive. Next select the "Create New Partition" option to install Windows which will prompt you to select size for that partition which will the the OS partition. Setup will then format that partition and start installing WinXP. You can either allow it to complete the install after which you can install the imaging program and then use it to restore the OS image file, or if you have the imaging program on bootable media such as boot CD, the install can be aborted to allow the imaging program to restore the OS image file after you boot to the imaging program.
After Windows is either installed or the OS image is restored, you can create a partition for the remaining space on the drive and format it after which you can restore the data image to that partition. Just make sure that partition is large enough to hold all the data originally on the 80GB drive. For example you wouldn't want to make it a 40GB partition if there are more than 40GB of data on the original 80GB drive.