Well,I know what you mean.but it's very hard today.

There was such a unpacker named UPC can unpack nearly everything for apps generated by Borland's compiler under DOS.

It works depend the facts that there will be a far jump to reach OEP of original app,and there is a patten for their initial code.

under windows,more and more technique have been developed to obfuscate the edge between the shell and the nut.There is a little code that can camouflage any app into a "VC++ compiled" app...