The way it works on most computers, when you “delete” a file or program (including emptying the “recycle bin” in Windows), it’s not actually instantly erased from the memory on the hard drive. It just means that the part of the memory that’s occupied by the file/program is marked as being available to be overwritten by new data if necessary. The deleted object becomes part of the “free memory” on the hard drive, in other words. This is why it’s possible to perform “data recovery” of deleted files, if you can locate the part of the memory that they were stored in, and they haven’t been overwritten with new data yet. It also means that if you really want something to be truly, irrevocably removed from your computer memory, you can use “file shredder” software to make sure that it’s overwritten (but even then, there are apparently some advanced data recovery techniques that might be able to recover the erased stuff, completely or partially).
What would this mean in the Tron universe? If we assume that “derez” is just another word for “delete”, it could be possible that the Programs we see getting “killed” might be “resurrected” through data recovery. When we see Programs fade away, maybe they’ve just been relocated to some kind of shadowy limbo in the system memory, awaiting their final overwriting by new data.
Take poor Ram, for example. When Flynn has returned to the Real World after his adventures, it might be possible for him to restore the friendly actuarial Program if he can locate the memory in which he was stored (which shouldn’t be too hard for a genius programmer/hacker, right?). He would have to be quick about it, however, since I imagine that deleted files could be overwritten fairly soon in a big corporate network with multiple Users like the ENCOM system. Once he’s gotten over the thrill of being back home and having defeated Dillinger, I’d like to think that he hurries over to a computer in the empty offices and does his best to restore his digital companion during the same night, since it might be too late once the building opens for business the next day. I’m sure Roy Kleinberg would appreciate it as well.
I also like to imagine what would happen if you’re not quick enough to restore a derezzed program, if their data has already been partially overwritten. Would they show up missing an arm or a leg, or a chunk of their head? Would their mind have been altered in some way?
This could also mean that the digital world has their own versions of “ghosts”, depending on what happens to derezzed-but-not-yet-overwritten Programs. Maybe there are stories going around about feeling the presence of dead Programs in places where they used to live, or seeing shadowy images of them in the corner of your eye, or hearing their voices whisper from far away.
Also, in this case a “file shredder” Program would probably be a pretty intimidating figure: A sinister “ghost hunter” who goes around the system, with the goal of making sure that the dead stay dead, their memories and secrets being buried with them.