Basically -- you can't -- because HP does not make their OEM Windows images available for download.
To get their OEM version, you have to purchase something known as HP Recovery Media ...
HP Recovery Media is a set of DVDs and a CD, or USB stick, that will erase the hard drive (removing all data, settings, and applications, reinstall the original OS, drivers, and some HP Utilities.
In some cases, you may be able to order a USB stick instead of disks. You have to order these from HP; they can not be downloaded.
You can look online for Recovery Media starting with the linked paged: http://support.hp.com/us-en/drivers
Once there, input your Product name or number. On your Software and Drivers Download page, select your Operating System and and Version. Click "Update".
If HP Recovery Media is available for your machine, down near the bottom of the page, you will see an entry for Order Recovery Media-CD/DVD/USB. Click the "+" symbol to expand that entry and click on Order Media for details.
Or, if you prefer, you can do the same by contacting HP Customer Service:
If you live in the US or Canada, contact information is on this page: http://www8.hp.com/us/en/contact-hp/phone-assist.html#section1
If you live elsewhere, contact information is on this page: http://www8.hp.com/us/en/contact-hp/ww-contact-us.html
NOTE: After you get through, stay on the line until you are finally able to talk to some one -- it can take a while!
If you have trouble finding a phone number, then try: 1 (650) 857-1501.
If HP no longer provides Recovery Media for your model, another site you can check is: http://www.computersurgeons.com
When you get the media, if it is a USB stick, insert it into your PC, pressing F9 to get a Boot menu, then select the External USB Hard Drive to boot from it.
