Thanks for your reply. (图)

I check the message prompt during starting up(the picture is posted below). It shows that it passed the memory test successfully. In my feeling, the PS/2 is likely to be the point of failure. I have 2 computers in use , I switch the KB and mouse quite frequently since just one set of KB and mouse avaible. It's not possible for me to measure the voltage on various pins coz I don't have the equipment. to buy a multimeter is not practical (too expensive, more than hundred$). It looks like I have to replace the mainboard for the defective ps/2 port - not a pleasure job to me. Btw, could you explain what is the picofuse you mentioned? where does it located on the board?