As far as I understand it, the key is independent of the installer you use, as long as the key matches the version of windows you are installing. You should be able to use any installer and then activate it with the key. Before you do anything, I would see about making a set of recovery CDs. If your backup OS is installed in a hidden partition, like is the case with many OEMs now, most of the time you can make a CD version of this to restore your computer in case something happens to the backup partition. This would be a safety net in case something goes wrong. You could also image your current OS partition with clonezilla so that you can restore it in case something goes wrong. At least you would be back where you started.
It is best to have your OS and apps in their own partition on the outer edge of the drive. Your data should be in another partition. This makes the reinstall process much simpler and your OS will perform better. Your OS images will also be of reasonable size. If you don't have this setup, now would be a good time to re-arrange your storage.
You should be able to download any drivers you need from the HP web site. Windows 7 is pretty good at not needing third party drivers, so you may be able to install and then use windows update to find the most up to date hardware drivers if you can't get them from HP.
If you want more detailed instructions on how to do anything above, just post back.
LMHmedchem