I think there is a bug in the windows version that can corrupt the database when you shutdown the client. You might want to try reindexing the blockchain or reinstalling the blockchain but use the same wallet, or try to export your private keys and import them into a different wallet such as multibit.
That sounds as if we should copy the blockchain onto an SDcard in case we have to re-install. I don't want to go through the wait whilst downloading again.
