hold control, shift and esc. check out the processes tab. sort by cpu (by clicking on the word cpu). now make it freeze up. see what gets the largest number in that collum. thats likely the problem.
and keep in mind that windows isnt like linux, it doesnt like to be running a long time. nor does it do well years after its been installed. i generally sum this up with "the longer windows runs the less reliable it gets". anyways it might simply be time to reinstall windows. thats an old processor so im betting your OS install is pretty old too.