I encountered an 0xc0000005 error (and event ID 1000 in application log), when launching a new installed Windows Messenger 14. Situation: Vista was reinstalled on a crashed Windows, and some settings and applications from the previous Windows installation were still causing some problems.
Just uninstalling and reinstalling did not fix it (more or less as expected, but you never know). In fact it was not possible to do a reinstall after uninstall, because the previous installation was still seen by the standard MS installation file. I removed all registry keys under HKLM/Software/Microsoft/Windows Live: no luck. Even moving all Windows Live … folders under Program Files did not help. Then I also moved the Windows Live and WindowsLiveInstaller folder under Program Files\Common Files: still the same, but after reinstalling Windows Messenger using http://ninite.com worked without problems. And now Windows Messenger worked perfectly.