I was placing my Contacts into categories when my WLM 9 crashed and
since I was Unable to login using WLM 9 beta, saying "service is temporarily
unavailable" etc..... (error 81000451)
Just what is this 81000451 error anyway?  I cannot find any information
about this error code.  Judging from the errors and symptoms I have been
experiencing, I now suspect it is related to some kind of corruption of
my Windows Live ID account database caused when my WLM9 crashed.

Is Microsoft investigating this error?  I see several people are also
experiencing 81000451 login problem and no one seems to know a fix or if
Microsoft is aware of the problem!

PLEASE FIX THIS ASAP!  THANKS!

Well since Windows Live Messenger 9 (WLM9) is beta it's not 100% stable and things like that can be normal expected. You should downgrade to 8.5 to be honest

However if you don't want to change read, here.

Also the search feature of this forum shows this. Which is also related   Good Luck

there is no fix ATM, so jut go back to 8.5 for now i been on the hunt for  fix for bout 2-3 weeks and "nada"