I just put my Power Supply back in my computer, to test and see if another Power Supply would fit it, which it didn't.
So, upon this mishap, I then put my old Power Supply back in, and reconnected everything to its original state.
I've yet to return the 465W Power Supply I bought that didn't work, but I'll get around to it, and buy another in the future.
But now, upon powering up my PC, the error "Reboot and Select proper Boot device or Insert Boot Media in selected Boot device and press a key." < Exact wording.
Here's the catch,
I've misplaced my boot up discs that my PC came with (By the way, I'm running Windows Vista Premium"
AND I've tried the (Ctrl+F10) way of a factory reset, but that didn't work.
Is there any way possible that I can bypass this error, and proceed to using my PC as I was before all of this?
I suspect you've dislodged a cable, or failed to connect one properly when changing power supplies. Check all cable connections are seated properly, not just the power cables but data cables as well which may have been disturbed.