How to take backup of a file which is allready created and open(running).

[Banks K]

Hello All, I have to take back-up of a file which is either of word, text or excel etc and which is already exists. For example: suppose there is a file which have 4 lines of text today and anybody came tomorrow and open this file writing something in this file and does not save this but my application is able to take backup of this file data which is already saved. How it is possible because I am getting only one method which is like read the contents of the file and write it into another file but it falls down the performance of my application. if anybody have idea how can I achieve this please share with me its very urgent.