my pc is very slow

I have a p 4 512 ram 160 sata hard and asus motherbord p48xmx machine. it was a very good machine.but now it is very slow i fomated and reinstall windows few times but it was not succeed.now it take about 10 minits to write a 650 mb cd and about 1and half hourse to write a dvd what can I do to solwe this problem. ram changing and diffraging was also not succeeded. need helpthanks all !