System Runs very Slow..

My System runs on WIn 98 . it was too slow i tried few diagnostics things like cleaning up. system was failing to run Defrag.

Finally i formated my C Drive & reinstalled Win 98. but the problem continues. system runs noramally after restarting for aboout 30 Minutes. then it becomes very very slow.

What is causing this slowness even after a neat reinstallation ??