SHELL MISSING

My computer has started sending me the error message,
SHELL.DLL not found. I look in my system directory and
it has a shell.dll listed? Is this the same? I have
several programs that will no longer open because it says
this shell is missing. I copied the file SHELL.DLL from
my other computer and put it in the sytem32 file and it
let me open a program, but then shut down and said
SHELL.DLL not found. I checked my system 32 file and it
was gone! I don't understand what is happening. I
checked my other computer and it has the file SHELL.DLL
in the system file. So I copied it there, but the
programs will not work, they will only open when it is
placed in system 32, but upon opening program,(which
immediately shuts down) it will once again say SHELL.DLL
not found and when I check it's gone? Does anyone have
any idea what is going on. Any suggestions?