There is virtually no way you can safely erase data from your computer that some day, using some tools can't eventually be recovered! However, there are programs that can make the recovery hard enough so most people without access to specialized tools or hardware will not be able to gain access to your sensitive information.
One of such programs is FREE ArtPlus File Shredder!
To shred your data, simply run the program, drag and drop files to the icon floating on top of all windows (as shown on the illustration below). You can simply right-click the icon when ready and choose to "Shred Listed Files" immediately, or "Open" the program to check which files are listed for shredding, and eventually adjust "Options" before shredding if needed. That's it!
PLEASE USE WITH CAUTION! YOU CANNOT UNDO SHREDDING!