ok just to make things clear about data deletion. When data is deleted, its is made not visible to the user and then the sectors get marked for Windows to write to those marked sectors whenever it needs to write data, there is a trace or a shadow of the data still on the harddrive till it get overwritten. So there is a possibility of recovery just as long as no new data it written to the harddrive.
** Things that people might now know **
Windows is very effective at using disk space, it will not write to blank sectors of the harddrive unless it has too, it will write to the sectors which the files you just deleted were stored on
Thus if you accidentally reformat your computer, Windows will install on the same part of the harddrive as your previous Windows installation, making recovery almost impossible with regular recover software
As far as actual data deletion, there is a free software called DBAN which overwrites you entire harddrive with 0 and 1 about 2 dozen times making recovery impossible
some recovery software i recommend is GetDataBack and File Scavenger