You can download an ISO file to make your own Windows 7 DVD.
Just make sure you get the one that matches what you have. If you have problems with that activation, then HP did not activate and send the info to MS. Something I have seen once, but again, MS did not give any issues with it.
You will NEED the COA key that should be on the system.http://www.w7forums.com/official-windows-7-sp1-iso-image-downloads-t12325.html
Please disconnect the hard drive when doing this install as if things do not go well, at least you can boot from that hard drive.
It is also a good idea to get all the drivers you need from HP or the makers website(video card) before and have them on a CD or flash drive.