If you have HP support contract, I recommend you to ask HP support first how to deal with Windows7 clean build.
How to build windows7 enterprise1. Download "Manageability -Driver Pack" from HP Support .
2. Extract the downloaded file to a USB memory stick.
3. Turn on your computer and strike F10 on HP logo menu then go to Advanced.
If the HDD size is more than 2TB, the partition style must be GPT, you must enable UEFI boot option in this case.
The others, you can choose GPT or MBR.
4. Check what status of UEFI boot option and Legacy boot option are.
For your reference, I proceed the setup process with
UEFI mode : Disable
Legacy BIOS-compatibility mode: Enable
5. Moving up CD/DVD boot order under the Legacy BIOS/UEFI what you choose in Step4.
Save and Exit from the BIOS setting.
6. Insert Windows7 Enterprise install media, the start windows7 setup.
7. After choosing language and keyboard type, Windows install wizard would detect some drivers are missing. Please insert the USB memory stick what you prepare in STEP2, then choose it as the driver locations. In this process, you might be required to load a storage driver in the USB memory.
8. Click custom (not upgade) to do the clean build.
9. Load another storage driver in the USB memory if the windows mentioned a driver is missing.
10. Choose the partition you want to install windows7 or delete everything then recreate new partition. In this process, you can find what partition type you have now. With UEFI enable on BIOS and MBR partition type, warning popup - you are not in recommended windows7 install with this partition type etc.
In that case, you are required to change the partition type, by strike Ctl + F10 key. Then convert the partition type along with this command.