You don't need a full version installer as you qualify for Upgrade version since Vista came with the computer, whether it is installed or not.
Back up your files and gather your program installers. Audit installed hardware and software including Product Keys using Belarc Advisor.
Backup C:SWsetup and C:Windows/System32/Drivers
to browse after install for any missing drivers or software.
When you boot the DVD to clean reinstall, the installer should see an OS and pass a flag allowing use of the Upgrade key during install. If not leave the key blank then after install do one of the workarounds given in the tutorial which Stormy posted to activate on a new or cleaned HD - the registry edit is easiest and takes only minutes.
Hopefully you made your Vista Recovery disks - if not see if you try making them now in All Programs. The Recovery partition will likely not run after Clean Reinstall although you can leave it until afterward then try cuing it up to see it it will run - if not you can delete it to extend Windows 7 into the space. If you have the disks you can also consider deleting all partitions during install using Custom install>Drive Options advanced. Illustrated steps here: Clean Install Windows 7
The installer is mostly driver
-complete, with newer arriving quickly via optional Windows Updates. Just in case, have your Wireless driver on flash stick or CD so you can get online quickly. Then enable hardware driver auto-updating
and check for optional Updates frequently.
Any drivers still missing in Device Manager can be found on the HP Support Downloads webpage for your model computer or device where there may be newer ones, or by browsing to any drivers files you backed up beforehand.
Install updates and then programs slowly over time to gauge performance changes after each. Don't let any programs write themselves into msconfig>Startup as they slow startup, become freeloaders on your RAM/CPU and can spy on you. I only allow AV and gadgets. Startup Programs - Change
Use a lightweight free AV like MS Security Essentials which works perfectly with Windows 7 Firewall. http://www.microsoft.com/security_essentials/
When it is finished, clean and order the HD perfectly using state-of-the-art free CCleaner Disk and Registry tabs, then Puran boot-time defragger + full Disk Check after adding Intelligent Optimizer on Additional Operations tab: CCleaner - Free software downloads and software reviews - CNET Download.com http://download.cnet.com/Puran-Defra...-75115626.html
Finally save a Windows 7 Backup image externally so you never have to reinstall again, just reimage the HD (or replacement) using DVD/Repair CD with the stored image. Backup Complete Computer - Create an Image Backup
You can extract HP apps and utilities from Recov Disks using this: Extract HP 3rd Party Software from RecovCD.