Hi:
If you can still read all 25 characters of the W7 product key on the Microsoft sticker on your PC's case, you can make your own plain version of W7.
This site claims to have genuine, unadulterated W7 ISO files for download...
https://www.heidoc.net/joomla/technology-science/microsoft/67-microsoft-windows-iso-download-tool
Click on the link labeled Windows ISO Downloader.exe
You can use either the:
Win 7 Home SP1 or the (Retail version, but the OEM key on your PC's case will work with it)
Win 7 Home SP1 COEM (This version would be for what HP would have used--System Builder)
N is for European Union (EU) countries.
K is for South Korean markets.
Then you can use this tool to transfer the file to a 4 GB USB flash drive or a DVD.
https://www.microsoft.com/en-us/download/windows-usb-dvd-download-tool
If you can't read all 25 characters of the W7 product key, you will have to contact HP for the country you live in at the link below and see if you can still get a set of W7 recovery disks for your PC.
http://www8.hp.com/us/en/contact-hp/ww-contact-us.html