Can't input "some greek character" in the current unibyte buffer -- so what do I do to change that?

[Mirko]

Hi,

I am using emacs 22.1 on windows XP.  Every once in a while when I set
the encoding system to greek, I get an error message when I try to
input greek characters. For example

Can't input α in the current unibyte buffer

I tried setting the encoding to mule-utf-16. But that did not help.

How can I enable this buffer to accept greek (and other) characters?

Thank you,

Mirko