Thanks to Jorge for this great blog post - http://blogs.technet.com/b/raymond_ris/archive/2013/04/04/cannot-open-or-create-word-documents-in-sharepoint-2013-with-office-2010-and-project-2013.aspx explaining how, after loading Project 2013 this can make your SharePoint libraries default to the 2013 versions of the protocol handlers. You might get a Url such as ms-word:nft|u|http://o15psvr:8080/Shared Documents/Forms/template.dotx|s|http://o15psvr:8080/Shared Documents for Word or ms-excel:ofv|u|http://sharePointDocumentlibraryURL for Excel.
The fix is easy – go take a look at Jorge’s blog.