Indeed, when something is deleted the sectors on the harddisk are simply MARKED as empty and free to be written to, rather than the entire content being deleted (most of the time the first few letters of the filename are gone under an NTFS,FAT32 partition)

This obviously provides us with the ability to recover files we have "deleted" ASSUMING that no application then proceeds to write to that "empty" sector on the harddisk.

I would assume (not knowing the particulars of how each file system works) that after deleting something incorrectly, but having no "undeleter" the best bet would be to delete MORE files that u really dont want as to prevent your particuilar sector with the files you require from being written onto. Not quite sure if this would work.

Even browsing the net etc using your computer generates temporary files that may be written onto your "empty" sector, be warned! (and that includes downloading these 'undeleters'