XOR encryption, as many have mentioned, is a toy. It might make you sleep better at nights, but that's only the blissful feeling of ignorance. If it was worth money, someone would crack it.

Why not just use GnuPG? There is a client for Windows, and this thing is sure to be about as uncrackable as you can get off-the-shelf.

Just remember to keep your secret keys secret, right?