HP as well as many other branded computers come with it's own odd internal design.
That sometimes limit the upgrade options. (looking at you DELL)
Here is some examples on branded builds:
This piece of disaster has it's harddrives in a non standard and hard to reach position and 180 degrees rotated ATX motherboard.
It has space issues and and a weird metal plate doing nothing but standing the the way of easy access.
The cable management is also a joke.
This disappointment is a little cleaner looking. But one thing you don't see on the picture is that it has a metal plate sticking out behind the PSU. That results in you have to use a selected range of DELL PSU's if you want to upgrade. (i have had this model my self)
It also has a non standard motherboard and RAM being blocked by cables so upgrading is a bit of a hassle.
Don't even get me started on this homemade bomb.
Now this is how a PC at least should look like.
Easy access to everything and it uses standard ATX. It doesn't waste space on unnecessarily things.
Or if you are an enthusiast that cares about looks, then check this beauty out. No explanation is needed here.