Del, If you get the disc from your friend, in addition to not using any disk that does not need a key, (a pirate disk), you should definitely NOT use any key they have unless you can be certain that they have not used the key previously. also as you have a Dell, you could use a Dell disk from your friend, but that would also have to be un-used. You should also not use a disc from any manufacturer other than Microsoft Direct or Dell.
The key may work initially to install but will likely never activate (or if it did, would normally be caught by Microsoft's regular checks so would not allow you to update the OS for security patches ETC)