ah...
when I was searching, I wanted to match the "uk" part, but couldn't really find it...but I think this is your closest match
HP Pavilion t3000 Desktop PC series*-* HP Pavilion t3000.de Desktop PC - Product Specifications - c00411711 - HP Business Support Center
so looks like you have a micro atx...which should be able to fit in m-atx and larger cases
HP and Compaq Desktop PCs*-* Motherboard Specifications, K8S-LA (Salmon)*
my hps that I have I have had to remove both sides...but they really have no standard with their systems, lol
as for your hiyatek...should be ok as well. Just take a look at the diagram in one of the links to see what the HP takes up now...you'll have to plug in some usb/1394/sata and maybe audio
though you don't have to plug in everything and just use it for the usb and fan control if you wanted