|Reset your CMOS through jumper or by removing the 'coin' battery (power disconnected) for a number of seconds. Then go into BIOS and reset to defaults.|
If you removed the CPU heat sink, did you clean off the old compound and replace it properly with new thermal compound?
A USB keyboard if connected to a rear USB port directly on the motherboard should be active at boot up and in your BIOS set up (Some front ports, all ports on add-on cards, and all ports run through hubs will not be active until drivers are loaded with Windows) from early XP machines through current machines.
Check all of your power connections including both motherboard plugs.
Check all front panel connectors for position and polarity.
Reseat all cards including memory.
Check hard drive cables.
Make sure all motherboard spacer posts are properly installed.
Post back results.
You have to be a little bit crazy to keep you from going insane.