Anyways, probably a stupid question but I've got this old HP Pavilion and I am planning on putting a new motherboard and Cpu and ram and whatnot into the old case (hopefully haha). Here is the page for the case:
The HP board that in there is a "Micro-ATX: 24.4 cm (9.6 inches) x 24.4 cm (9.6 inches)" I've upgraded a few of those systems, there is *NO* room for a larger MB in the ones I've worked on.
Aside: I agree with the people suggesting you also get a new case. Having the right parts to install the MB will make your life much simpler. And the old HP system if it's still running is worth far more than the case will cost you. Give it to a friend or EBAY it.