>I want that my applications should seamlessly encrypt/decrypt all the files (DBFs, CDXs, FPTs. One of the ways i think is to replace the DOS interrupt functions at the start of the application and reinitialise them to the origional state when the application quits.. I will welcome advice if anybody has tried this option.

Instead of doing this, I will suggest you to use ready-made softwares available for that..(like cipher, cryptor etc.)