There is nothing on any PC that can't be deleted, outside of the OS, of course.
Removing all users except the Admin (me) gets rid of everything the old user did and all the files he/she glomed onto.
Then running a good UN-Installer like Revo Uninstaller, not only gets rid of unwanted programs but cleans their leftovers out of the registry too. Then after removing all the junk out of the registry, it's a good idea to re-compress it with a safe program like NTREGOPT.
Wiping and reinstalling the OS on an old PC is fraught with pitfalls, like getting the drivers for all that old hardware.
Been there and done that and it still makes me shudder.
On certain brands and models, that's ni'on to impossible.
I just clean them up and then make a Ghost backup DVD for that PC, just in case that old HD shoots craps.
It's like having a factory restore CD, that can put the computer right back the way is was when I got finished with it.
I guess each guy has his own technique, but that's mine.
(Not meaning any disrespect to anyone who wants to do something different.)
Just a thought, from one old Tech.