eddiemor, welcome to the forum! If you want to keep the OS that was installed by the OEM (HP), you certainly can keep it. What you need to do is use an imaging program such as Macrium Reflect (can be found in the Free Software Database on this forum, with a description of the product). Back up the entire drive ( the OS, the 100MB system partition, and recovery partition). Prior to doing this, you may want to burn recovery discs, this option should be in the Start Menu. Just type "Create recovery discs" in the box, and the program (Power2Go, with HP) will allow you do this. You'll need three to five quality DVD discs for this purpose (consult your owners manual for details, plus details on burning the recovery discs).
As far as using the same Product Key to activate another OS, you can't do it. That key is only meant for the OS that was preinstalled on your system, and when you do reinstall it, you won't have to enter the key, it's already preactivated for your OS. You don't have to remove the OEM key that was supplied by HP, as long as you have the OS backed up, and/or have recovery discs to reinstall it with.
If you want to install another OS on the drive, you'll need another key for what you install, it should be supplied with the other OS that you have.
Hope this helps.
Best of Luck,
Cat