i am back and solved the problem myself, actually it not was difficult in vista just deleted the entries pertaining to virus using regedit as regedit was not disabled ( in case of Windows xp regedit was disabled) and the scanning with the anti virus Dr. Web , only anti virus to catch the Ahsn virus and problem solved.However in case of windows xp it wasn't that easy as i was not able to instal applicationand regedit was also disabled,actually solved by problem by booting the system from a xp live cd and removing files and registry entries manually and also took the help of RRT.Any expalins welcome