Like GmsCool stated, the problem is with your case, not your PSU, which is standard ATX sized.
I know some Antec cases use PSU brackets to mount the PSU, so your case must be one of those. Their customer support is usually pretty good about replacing bits and pieces of the case.
But if they don't, you can always try some small mods, like wedging a block of wood to keep it in place or using rope or something to tie it down.

HP Media Center machine
MOBO: ASUS OEM board
CPU: 2.6 GHz Socket AM2 Athlon X2 5000+
RAM: 2 GB Qimonda PC2-4200 DDR2 RAM
GFX: Sapphire Radeon HD 4850
HDD: Seagate Barracuda 320 GB SATA 7200 RPM