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:
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Reboot
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Click START>Run and type 'command' to load the console.
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Type in:
regsvr32 wups2.dll
and press enter.
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Type exit to leave the console
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Download and run the windows live installer, it should now work perfectly.