Most major-brand PC makers (HP, Dell, eMachines, etc) use motherboard and power supplies that have odd shapes or mounting hole patterns. You can often "force" something else in with cutting or drilling, but you normally can't order an industry-standard component and expect to just attach four screws and go.
I consider major-brand PCs to be like kitchen utensils. Use them until they wear out or become obsolete, and then trash them or give them to your neighbor. If you have to pay for repairs, you'll often find it cheaper to buy a new one.