Excel stores the string on the clipboard using the Unicode character encoding. The reason you get a square when you try to read the string in ANSI is that there is no representation for that character in your system's ANSI codepage. You should just use Unicode. If you're going to be dealing with localization issues, then ANSI is just more trouble than it's worth.
Edit: Joel Spolsky wrote an excellent introduction to character encodings, which is definitely worth checking out: The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!)