OK step by step.
Go to BIOS and see if its recognizing HDD. Do a check disk. If all's well see below.

• If you want to format the drive and put a fresh OS, you will need to get the manufacturer specific version of windows 7. Do a little research on another computer and try finding a specific iso for the computer if you cant, go to next step
• Download windows iso from http://forum.notebookreview.com/windows-os-software/604187-legal-download-digitalriver-windows-7-sp1-13-languages.html
• Select Home Premium, Professional or Ultimate. You can look at the Certificate of Authenticity label to determine which one it is.
• Next Step: Also download the tool that converts the iso image to bootable usb or dvd. You will need about 8 gbs.

After you create bootable usb/cd

• USe the boot device priority to select CD or USB drive as default boot partition.
• Insert the media and then restart. If you successfully created a boot drive, it will load the files from there.

After the files are loaded follow the steps. When you are asked to put the keys, your OEM keys will not work. Just continue without the keys. Windows gives 30 days for activation.

To get another set of keys you will have to call Microsoft. Here is an article:
http://pcsupport.about.com/od/osproductkeys/ht/windows-product-key-replacement.htm

Once you get the key you can go to computer>properties and there is a box that says change my product key.

• After this go to HP's website and get all necessary drivers.
• Once done Get Ninite and install all required progs.
• After that update Windows.

Once all the updates are complete, Go to Backup and Recovery and create a system image. This will help you for the disaster next time
Put questions with screen shots if you have problems.