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

Update: there is also another fix (Thanks MeggarK) which runs as follows:

1. Download and run the Windows Live Installer Cleanup tool from:
   
   http://download.microsoft.com/download/2/3/0/23036331-516d-4c7e-b9ac-31554a2fec06/WL-Beta-remove.zip
   
2. Reboot
   
3. Click START>Run and type 'command' to load the console.
   
4. Type in:
   
   regsvr32 wups2.dll
   
   and press enter.
   
5. Type exit to leave the console
   
6. Download and run the windows live installer, it should now work perfectly.