Read this, I think it can help:
Error Code 0x80070643: Cannot install Windows Live Messenger 9 or other Windows Live Applications
Or try to go to Start>Run, type 'cmd' and then enter each of these one at a time at the command prompt (hit 'enter' after each one if needed):
regsvr32 wuapi.dll
regsvr32 wuaueng.dll
regsvr32 atl.dll
regsvr32 wucltui.dll
regsvr32 wups.dll
P.s. Did you try the full installer
http://g.live.com/1rebeta3/en/wlsetup-all.exe ?