Hi,
It seems that I forgot to mention a few points,all the workers of my client work on windows.So to prevent any transfer of files via internet we removed (disabled) the internet access from there.My client is a Charted Accountant and all most all his work is done in accounting s/w like tally.Internet access for his workers is not his immediate concern.

So even if one worker wants to access the internet he has to reboot the computer into linux and then access it. (XOSL)
In this case he will not be able to access the drive where the senstive data is kept.
And as mention above we we remove all the USB,CD RW ,Floppy drives and put a lock to the computer case.

But then there are some smart chaps who have mastered the fine art of lock picking.