New CPU no input signal and no power to keyboard and mouse.

What did not have any problems before?
You mean with the old CPU?
Remember the Motherboard thinks it has the old CPU so you need to clear the bios so it can discover the new CPU.
You did check to see if the bios supports that CPU? You might need to update.
and that the mother board supports that CPU voltage wise.
and the power supply can provide enough current?
HP's are built with PS with just enough power to work as supplied, so you might need to get a replacement.
I would start with HP website.
Now if it was working with the new CPU but now doesn't it can still be not enough power.

No, there were no problems with the old CPU it was a single core processor and the new one is a quad core, it was an upgrade. How do I clear BIOS?

I inserted the old CPU but I still do not get any video signal or power to my keyboard or mouse.

what socket is your mother board ? can you post a model number or serial number for the motherboard?
there could be an issue where you have an older socket 939 or earlier and your trying to replace it with a am3 processor .. your motherboard will need to be at least and AM2+ for that processor to work and that saying that particular motherboard supports that processor. also as the previous poster noted HP computers are notorious for having bare minimum power supplies in them you will more then likely need to upgrade as that processors TDP is way higher then the previous model and you also may run into issues where your OEM license will not accept the new processor and force you to re-purchase another license of windows

I have an AM3 socket. I have a 450 watt power supply I will put that in and try.