Hello, I power up computer and cpu temp immediatly climbs to 100+ C, then shuts down. tried ps,ram,cpu,hd from good system and it does the same thing. is the MoBo fried? what reports cpu temps?
Do you have the heatsink properly mounted? Is the thermal paste fresh? Once it has been used it cures and must be cleaned off and new stuff must be applied for it to work properly.
Check the heatsink and the obvious first, but don't discount the possibility of a dodgy sensor. 100C is so hot you should be able to feel it. Try taking the side panel off and then booting; you should be able to feel the heat if it's that bad. Good luck!