Gpg4win is a gui front end to GPG (Gnu Privacy Guard).  It is used to encrypt, decrypt, and digitally sign files.

It’s compatible with PGP.  I managed to load my way old PGP into it.

It’s a shame that encryption things like this are not well integrated into email programs.  It’s more of a copy/paste operation in order to get encrypted mail working.  Especially with web-based mail like gmail.