Did you do something recently that may have messedup with your winlogon.exe? It had happened with me once due to that reason.
Are you able to boot into safe mode?
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arre yes, i forgot that. Ya also check what Choto Cheeta said. It could be heating etc. problem
usually dust is the main problem with older system causes the FANs to spine really slow and dust on products it self traps in heat within hence forceing heat up the system
I checked the SMPS and even checked by changing the SMPS. Still facing the same problem. So I decided to format the PC and after formating PC windows XP is not completing in middle of the setup PC Shutdown. When I restart the PC it give Checksum error, press F1 to continue then I changed the CMOS battery then also I am getting same error and even windows xp set up is not completing. What to do please help.