Try looking into MSDE the free version of SQL Server, or better yet, try SQL Server 2005 Express. This have the full blown power of SQL Server, but MSDE has a limitation on concurrent connections. Overall, MSDE is a great DB engine and not too hard to learn. Do a search on MSDE on this site or Microsoft site.
I heard that Disney is using MSDE with a .net front end and something like 200 users. It is strong once you know the limitations. Just make sure to close the connections as soon as you get your data or do your data edit.