There are two options which incorporate the good suggestions which have already been made:
1) Run HP System Recovery
from it's partition at boot, if you want the preinstalled factory programs and bloatware which can be the very worst on HP's. But check if there is a Minimal Recovery choice on that HP once you boot into Recovery.
If you run Recovery you can mitigate some of the bad effects which throttle Windows 7 using Clean Up Factory Bloatware.
2) As suggested follow the steps to get a perfect Clean Reinstall - Factory OEM Windows 7
which include everything you need, including Special Notes for HP owners.
Ask back any questions to help decide, or as you go.