Even if they are deleted, they are still SOMEWHERE on the hard drive, as many a crook has learned to his dismay. Also, the Geek Squad charges fairly steep fees for “erasing” stuff from hard drives before used computers are sold or donated.
The basic answer is "it depends." Servers usually use arrays, which are more difficult to recover since you're searching for bits across disks. Depending on the usage, the data could have been overwritten several times already. In the end it depends on the specific case and how much time and money you're willing to spend recovering the data, and it can be a LOT, especially if you have to go the electron microscope route. There is academic research in this area to determine the wiping patterns that can best disrupt the magnetic fields, so it's getting harder.
Also, the Geek Squad charges fairly steep fees for “erasing” stuff from hard drives before used computers are sold or donated.
Funny, all they do is let a program run on your hard drive overnight. Even that isn't perfect though, although like above the time and money it would take to recover the data is insane.
May I suggest a free tool to do it yourself and save money, http://sourceforge.net/projects/eraser/. If you can make a boot floppy, wipe the whole thing with this http://sourceforge.net/projects/dban/.