There should be no problem installing a new Hard drive in a HP system to replace an old drive.
So let me get this straight, you removed your old hard drive, installed a new one and then installed your OS onto the new drive, but it will not boot, right?
If so, did you remove the OS disk from your cd drive after installing the new OS?
Are the jumpers (if it is an IDE drive) set correctly? If there is only 1 HD installed, then you would want the jumper set to either master, or CS (cable select).
If this is a sata drive, then you should just make sure the connection is correct.
Are you able to go into your bios? If so then set your first boot device to the hard drive and not your cd/dvd drive.
If all of those fail, you might have to start over with the OS install. Maybe something simple failed when installing it and it needs to be redone. If you do, make sure to reformat the hard drive again and do a full install of the OS.
I hope 1 of those things work for you, if not, come back and someone will try to help.