I tried to upgrade the cpu to an 850mhz, but I think the CPU is bad, because nothing happened when i tried to turn on computer. So I replaced the old 750 back in it. Now it turns on but it will only work for about 5 seconds, then locks up. I can get into bios but then freezes. If i just let it boot up normally it freezes. If I hit the ctrl button to select safe-mode or last known good config it freezes. Did i screw up computer by putting bad cpu in it? The other possibility WIN XP said i needed to register my copy of it, but when I put in the key code it denied me, and gave me 30 days to do it in. Well the 30 days was up the same day I tried to change the cpu. Do you think this could be causing the problem? Thanks for all of your help!!

it shouldn't be affected by the windows activation. It'll just lock your pc (it won't be able to make your hardware unbootable. Make sure all the connections are connected well, cpu, etc. You might have disturbed something, or forgotten to put something back.

I have taken out all the cards and put back in. Made sure all connections were connected. It locks pc before it has a chance to boot, keyboard and whatever point it has got to on the screen. Does that make sense? Am I not being clear on what is happening? Please advise.

Unplug the machine and remove the mobo battery for 30 seconds, replace it, plug in and go to the bios and set it up.

make sure the thermal paste has good contact

I agree with the other guys but I must ask what do you see during that 5secs? Also check the bottom (inside) of your PC case make sure no jumpers fell off (small plastic square) these can be used on motherboards to set many many things and could cause your problem if one came off. AND make sure no dust bunnies came off your CPU fan into the cpu slot,