I'm looking for software that will password protect or encrypt files,
so I can send them to a friend.  I need to encrypt of pw protect MS
Excel and Adobe Acrobat files.  The catch is that I have a Windows
machine and my friend has a Mac machine.  The easier to use the
better, as I'm not very tech savvy.

You could use PGPi http://www.pgpi.org/ I don't know if it allows you
to encrypt individual files but I think it does. What it does have is
an email plugin that you could use to send your file as an attachment.
There are versions for many OSs including Windows and Mac.

I have never used it so unfourtinatly I probably won't be able to help
you install it.

I have no idea if Excel or Adobe do this, but Word has a password
protect funtion hidden inside it. I would suggest searching the help
menu if you have not already done so.