will the phd450 fit my compaq 2800?

I have an LG which is a very tall gaming system and I had to really take out all the slack from the wires but it fit. It's not the quietest power supply but for the price its fine.

will this power supply fit my compaq 2800? phd 450

Very unlikely. I don't know that computer model specifically but when I Google search for "compaq 2800" it comes up with a laptop. This power supply is for a desktop computer. It fits a standard ATX case.