The part of Rex M's answer about using SHA1 to generate a hash is a good one (MD5 is also a popular option). zvolkov's suggestion about not constantly creating new crypto providers is also a good one (as is the suggestion about using CRC if speed is more important than virtually-guaranteed uniqueness.
However, do not use Encoding.UTF8.GetString() to convert a byte[] into a string (unless of course you know from context that it is valid UTF8). For one, it will reject invalid surogates. A method guaranteed to always give you a valid string from a byte[] is Convert.ToBase64String().
