Checksum the entire PC

I was wondering, just like there are individual file checksums, like
MD5, is there a way to checksum the entire PC?

Why do I want to do the entire PC? Because a single operation of
doing so is probably easier and faster than doing individual MD5
checks on hundreds or even thousands of files.

I'd like to know the state or signature (if you will) of the entire
box before any development or testing occurs on it.

Thanks!