Sorry for the dumb question as I'm a newbie. If I wanted to build a commercial Windows application (one I'll sell for $), what are my options in terms of an embedded datastore? Would I use MSDE? Is this something that I'd bundle with my app? Are there other options (both from MS as well as other vendors)? Do you have to pay a fee to the datastore vendor for each copy of their product that you include with each copy of your application that you sell?
Thanks.
One of the great choices for this is SQLite. It's completely public domain, so you can use it for any purpose whatsoever (including commercial software sold for profit) without any worry about licensing. There are bindings available for many different platforms and languages.
How embedded does your data need to be? If it's in-process then SQL Server Compact Edition would be a good option.
Following-on to @Matt's answer, if it does not need to be in-memory, but does need a small footprint and a server dedicated to your application, then you should consider SQL Server Express Edition, which is free.
Firebird can be a good choice too.
It have more features than SQLite