What will you be doing on the PC? Is it the stock HP PSU?
EDIT: Just googled the PC and there's 3 versions of it. Which one of the 3 models is it? : http://h18000.www1.hp.com/products/quickspecs/12253_div/12253_div.HTML
Some below which have the low height option:
http://www.overclockers.co.uk/showproduct.php?prodid=GX-116-HT&groupid=701&catid=56&subcat=168
http://www.overclockers.co.uk/showproduct.php?prodid=GX-233-AS&groupid=701&catid=56&subcat=938
What connections would you require.