When you start Windows Messenger or MSN Messenger you cannot sign in, and you get error code 0x80004005 along with the message:
Signing in to .NET Messenger Service failed because the service is temporarily unavailable. Please try again later.
This error means there was a general failure, which amounts to "there was a error but what went wrong exactly is unknown".
A common cause for this error however is that files that Messenger needs are missing. Try to reinstall MSN Messenger and see if it will work again.
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