I'd expect you to be able to open MS Word documents with WordPad, an application that's included with Windows 7. I don't use it much, because I have MS Office on the machines that I use. Office includes Word.
There are free/open source alternatives to Office, like LibreOffice, but I haven't tried them. I wouldn't expect them to support 100% of Word's features, but maybe they handle the ones that most people actually use.