System occupied 50% of system

Hi,
I have an intel main board with 3.0 GHZ CPU and 1 GB RAM, my desktop was very slow and always I see that the system occuipied the CPU more than 50%, I reinstall the windows XP it was so slow also and aftre that the same problem happend again and CPU is under perssure, I change CPU, RAM and format the hard drive (only partition C) but nothing changed.
what is wrong ?

Ali