I don't know how, but it started unexpectedly and suddenly. Tried everything from my side but nothing works.

I have Windows XP Professional Version 2002(i mean no service pack)

Well the problem is that whenever I press CTRL + ALT + DEL a error message is displayed with this content:

[B]TITLE: Winlogon generic control dialog: winlogon.exe - Application Error[/B]

The exception Single Step
A single step or trace operation has just been completed.
( 0x80000004 ) occurred in application at location 0x00000000.

Click OK to terminate the program.
Click CANCEL to debug the program.

Whatever I choose same thing happens. First my XP theme appears like Windows Classic theme(like previous version) followed by a restart without any Logging Off or Shutting Down message, I mean it abnormally restarts.

Also, the odd thing is that my computer works normal, until I don't press OK or CANCEL, but yeah the dialog remains at the top. Also once the error dialog is there, Task Manager open whenever I press CTRL + ALT + DEL. But clicking at any button of the dialog means restart.

Please help me solve this problem.

This sounds like something damaged the OS or an application. Reinstall.