If you're still having problems installing any of the new Windows Live betas (Mail, Messenger and Writer) and our quick/dirty fix didn't work for you, you might want to try giving the more official fix a try. Posted by the Writer team on their blog, the solution consists of running a command-line application that is available for download from the Microsoft website. Additionally you can email your log files to the team to help them troubleshoot the issue.
Writer Blog - Installer fix