I was busy with an installation of sitecore through the sitecore instance manager (SIM), asnd everything seemed to be ok, but I had no databases.
I gave Network Service read/write access on my c:\websites folder, but when I looked the SIM logs (Home > Bundled tools > SIM logs) I saw the error:
System.Data.SqlClient.SqlException (0x80131904): Unable to open the physical file "C:\websites\mysite.dev.local\Databases\Sitecore.core.mdf". Operating system error 5: "5(Access denied.)".
When I gave Network Service full control on the folder c:\websites it worked.
Install the latest version of SIM from http://dl.sitecore.net/updater/sim/.