how can i undelete files on a disk using a hex-editor?

i'm searching google for it al day now, but haven't found any good tutorials or anything...

i've read somewhere before that windows changes the first couple of characters of the filename into something, and therefore the file is marked as "overwritable", so when i know how to undo this, i could undelete files using only a hex-editor, and that's exactly what i want to learn!

does anybody know something more about this, or perhaps a link to a tutorial???