Hp pavalion desktop
There's your problem. Those things, along with most other made-for-retail desktops, don't have cases and component layouts designed with enthusiast graphics cards in mind. You would have to replace the entire case with a roomier one, but there's also no guarantee that the HP motherboard was designed with any other case in mind: if it has some sort of proprietary front USB/power button/LED connector on there (which some do) then you can't move the motherboard.
You may very well be screwed unless you contact HP and ask if it is possible to do what I am suggesting, if they say it is okay.