Keyboard and mouse problem

Hello,
I have a problem with Dell optiplex 755 computer,using windows xp professional service pack 2 dual core processor 1.8GHz @ and Ram of 512.
the pc boots very well but atfter boot the keyboard and mouse become inactive. But before boot process completes you can enter set up using the keyboard.
I tried entering safe mode it does but still the keyboard and mouse lock up.
what should i do? it's driving me nuts. thanks