windows xp starting up pc

for some reason when i start up the computer yhe cursor does not respond to
moving my mouse.
i have to switch off the computer, and on again in. the cursor works every
2nd time i switch on my computer. i have checked all the possible faults but
no success. incidentally, i revewed my mouse recently and now use a round
shaped portal plug (ps/2 )? instead of a usb plug. it is since i changed that
the problem began. any suggestions?? thanks.