Every once in a while I come across an old piece of email, or a document I archived that contains a little nugget; well, I just stumbled on one on a backup DVD. Last year, Microsoft made available a tool named the File Checksum Integrity Verifier (FCIV) to compute file hashes and perform comparisons against a baseline at a later date. It's simple and easy to use. And free.

http://support.microsoft.com/default.aspx?scid=kb;en-us;841290