Whenever I try to login in my winxp machine it shows welcome and then shows saving your settings and then logs out immediately.I am not able to login in any account(nor administrator nor user).
Please help me

...i am not able to login...

See if either of these match your situtation:
You cannot log on to Windows XP after you remove Wsaupdater.exe (http://support.microsoft.com/kb/892893)
Unable to log on if the boot partition drive letter has changed (http://support.microsoft.com/kb/249321)

Do you know how to start in safe mode?

Try that, and reset the administrator password.

If you need help with safe mode, let me know.