I need a RDBMS that satisfies a few criteria. I wonder if there are any suggestions.

I'm developing a Winforms application in VC# 2010 EXPRESS under .Net V4.
I use both a Win XP Pro SP3 desktop to do that and a Vista Home Basic laptop. The App and database will initially be fielded on such systems. The environment will be for small professional offices without a server. If the App becomes successful, it may be adopted by larger companies and it may be adapted to use their in-house RDBMS.

With the RDBMS should come drivers that allow me to connect to the database via C# code. It would be useful to have the tools that integrate with so that - for example - one can view in Visual Studio a table out of a DataTable or DataSet. Note that I'm using the DBProviderFactory class that makes generic use of connection, command, and command builder objects.

Of course it should be free too develop with and distribute.