I learned that passwords used to lock Excel spreadsheets and perhaps other microsoft document files are very easily hacked.

I downloaded a file that required a password, so I fed it through my password-cracking software. The key was generated instantly without the usual bruteforcing procedure that would be required for things such as rar files.

It was a long (20-30?) alphabetical key that didn't consist of intelligible words, so the conclusion must be that the file type has a weakness somewhere.

Lesson: If important, lock the file in an archive instead of passwording.