Deleting a file doesn't actually remove the data from a computer. To ensure your drive doesn't contain any personal data before you give it away or sell it, you need to reformat the hard drive or use digital shredding software if you want to completely eliminate all traces of data. When it comes to data stored on your computer, deleting files doesn't actually remove the data. File information is kept in a directory so that the operating system can find it. Until that region is overwritten, the old data can be retrieved, in fact that's how you can recover lost data.
Macs come with data wiping tools built into their systems. To securely delete a folder or file, all you need to do is move it to the trash can and from the Finder menu select 'Secure Empty Trash'.
Windows: Data Wiping by Selected File/Folder.Eraser is a free, open source program that works with Windows 95, 98, ME, NT, 2000, XP, Vista, Windows 2003 Server and even DOS. It uses carefully selected patterns to overwrite your hard drive several times and lets you select single files, entire folders or the entire drive to be wiped clean. Download and install the program, then select the files/folders/drives that you want deleted, and the method of deletion, you can run Eraser. read the article for more methods.