I would recommend the SQL Express, formerly known as MSDE in that it is the free version so if you do not have to run on compact edition (and why would you run a database there), then use the most available of the MSSQL's if you don't have a budget to buy another, it is quite good
http://msdn.microsoft.com/da-dk/evalcenter/dn434042.aspx[
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alternatively consider Microsoft Azure SQL which has intersting alternative offerings for a database or even MySQL which is very able and fast and free