As I remember from sketchy A Level computing lessons, last year with a bad hangover, when something is "deleted" from a disk it is simply flagged as erased, and is only totally irretrievable if it is overwritten. This may be total rubbish, but I've always found this an interesting question and I do believe there are companies out there who can retrieve such data. By all means correct me if I'm wrong here!
By the way, am just about to attempt rerunning the OS installation with a fresh stick of RAM...will give you an update on the results!!!