hi nabanita dutta,
Welcome to the digit forums!
Regading your problem I think your antvirus itself is infected. if the installed antivirus are not helping you out you can always opt for online scanning.
They are quite good when you anitivirus program itself is infected and ofcourse the online scanning uses latest updates and are free.

so go ahead and
scan it online
after you scan ur sys online and remove the u can install a fresh copy of ur fav antivirus.
and if ur question was to remove the nag u have to manually delete the antivirus folder and run a registry scanner. You need to delete in safe mode as the start up programs are not loaded in safe mode.
..:: peace ::..
Jeba