MySQL is a very robust, powerful, lightweight database solution. It lacks some features that SQL Server has (Stored Procedures, Triggers, and Views... that functionality is coming in version 5), but it's main feature is that it is open source! I use MySQL for everything I do. It is incredibly stable on a Windows platform, and its popularity is growing.
Use it as you would MSSQL. Let me know if you need any help. I've been developing ASP apps with MySQL for a few years now.
D8