When you remove a file in your computer in many cases it is transferred to the recycle bin. Now it is possible to either delete it, or remove it. Deleting it may give you an impression that it is removed permanently but it isn't like that. In many cases the moment you delete a file, all signs of the file may be gone, however only the header is deleted, and important files portions are still there. Those remnants are easily restored by data recovery systems and the data completely retrieved and used for many things.
That's the reason why data cleaning systems have become very common among average computer owners. Those systems range from free data cleaners that scour the inside of your OS to remove the unneeded files. To powerful cleaners that completely obliterate your files by deleting the files, wipe the fragments clean, overwrite the exact hard drive sector, remove the overwriting files, over and over again. The later phase can take many iterations from a few hours to a few days dependant on the extent of data cleaning that you need.
It is can also be done by swapping 1s and 0s in the hard drive and overwriting all files earlier saved on it. The more overwrites mean far less possibility your files can be retrieved. The systems should be usable despite the file format and OS used.