One possibility is that your new computer has a newer release of Word than was used to make the files you are opening just to read (I'm assuming these are files you created on your old machine and copied onto the new machine). If this is the case, the new machine is reading in your old files as being derived from an old version of Word, and it's essentially asking you if you want to save them as being compatible with your new version of Word. New versions of many programs (Excel, Illustrator, etc.) often cause this issue for me. If this is what's happening, saying yes to a 'save changes' should prevent the message from occurring next time that particular file is opened.