Is it possible to somehow use shortcut keys to paste in sentences? I am a teacher and would like to use this to quickly paste in a variety of comments without always having to copy and paste from elsewhere.

Look into AutoText and Building Blocks. Shortcut keys can be attached to them or they can be inserted by typing (a part of) the name and pressing the F3 key.
Automated Boilerplate Using Microsoft Word

You could save textboxes with different comments as either AutoText or as a Textbox Building Block. These would float in front of text rather than pushing it out of the way and could also be set in different colors and fonts.

Note, I do not believe that Word 2013 is supposed to work on Windows XP. I know I couldn't install it on Windows Vista.

Oops my bad - it should say Windows 7.

Thanks for your response - I've had a bit of a nose around Building Blocks (never heard of this before!) and it seems an avenue worth exploring.