Problems with CPU Usage is 100

Hi! My problem is with CPU, the usage in Task Manager shows 100% when playing games or using office.. I tries to close heavy programs in task manager, didn't help. Motherboard Gigabyte 8LD533 1.x, CPU Intel(R) Pentium(R) 4 CPU 2.40GHz. I tried to install INTEL 02 CELERON 1.80HZ/128/400/1.75V but still CPU 100%. Will appreciate your help.
