Last time when same problem occured ,i replaced its processor, but now i did every thing not working properly.
other parts are P3 600MHz ,128Mb,20Gb Quantum
i need repairing level help

Let me get this strait ,your computer wouldn't boot ,you changed the processor ,and it started worked again ,,but has since stopped working again .!!
How long did you computer work after you changed the processor .correct me if iam misunderstanding you problem !
also you say other parts Are: . P3 600mhz ect ect .the p3 600 is your processor ,We are talking about 2 different ones right .were they the Same type and size processor .maybe the new one isn't compatiable with you motherboard

2 weeks 12 hours a day it worked properly when i replaced last time its processor. I just changed this processor which was working properly(400Mhz with 600MHz).
after this it did not work with last 400MHz pocessor
now what to do

if it was hardware error it would probably say on POST, have you changed the mobo or flashed bios?? as if you change the mobo on a win xp machine it will quite poaaibly get to the windows boot crash and restart.

Could be lots of things ,video /Ram /motherboard ,
First Does the computer bootup at all ,do the fans run or is it completly dead .
just what happens when you turn it on!??

I thinking it could be the Motherboard or Powersupply ,But Ram and or Video Card can cause this sort of problem also .
If i read you right you are saying that the 400 worked ,you upgraded to a 600 and its now not working even when you put the 400 back in .Right .

If that is right ,you could try taking out the Ram ,the video card ,sound card ,modem and any other Cards that are in the Slots .and try turning on the computer to see if it powers up

You say it restarts when the Windows logo appears, have you pressed F8 before the logo appears and select Safe Mode? If it will boot in safe mode then it is likely a driver issue, have you added anything recently hardware, or eve software wise?