Hi,
This is very common cause in windows vista. Please have a look at http://www.vistax64.com/vista-mail/176160-windows-live-mail-wont-open.html and at http://www.eggheadcafe.com/software/aspnet/30631347/mail-database-error.aspx replied given nicely below:
Sounds like a bad install or something has gotten corrupted.
Try un-installing and re-installing the application. WLM uses a number of
data stores and without knowing which one it's going to be hard to fix.
You could always delete (or rename) your Store folder (with WLM closed).
WLM will then recreate the Store folder upon restarting. You will have to
start from scratch with all your accounts, newsgroups, feeds, and emails
though, as they will all be gone from WLM. I have decided that despite WLM
having good backup functions, I will still regularly back up my Message
Store.
-Paresh