Since this is highly personal / private info being kept track of, best thing to do is use Excel's built-in encryption method or use PGP to encrypt the file. www.gnupg.org is a good PGP tool to use.

In addition, I put a copy on a USB key that also contains my email addresses, bank account #'s, FF #'s and all pertinent login details. That goes into our safe deposit box so my wife has all this info if something unfortunate should happen to me. It's usually a month or two out of date since I don't go in constantly but it's still better than having no info at all.