Hello, Miguel,
if you want to see more details you can find it in event log which I bet definitely says that the user does not have sufficient permission for the database.
If you change the account under which you run the service and put there windows account under which you installed SQL server (assuming Windows Authentication) it will run or you need to give Local System account permissions to access SQL server .
Regards,
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Zdeněk Kabátek
Head of Professional Services
NEOOPS
http://www.neoops.com/Europe/Prague
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