When a file is deleted windows only marks the file as deleted by changing the first character of the filename to a funny character in the directory listing. Unless the hard drive is almost full then it is unlikely that windows will overwrite those files right away. There are free programs that can recover them. It all depends on how hard they want to recover your files. You can use a free file shredder like eraser found at:
http://www.heidi.ie/eraser/