I\'m writing an application that takes user data and stores it locally for use later. The application will be started and stopped fairly often, and I\'d like to make it save
A fourth option to those you mention are binary files. Although that sounds arcane and difficult, it's really easy with the serialization API in .NET.
Whether you choose binary or XML files, you can use the same serialization API, although you would use different serializers.
To binary serialize a class, it must be marked with the [Serializable] attribute or implement ISerializable.
You can do something similar with XML, although there the interface is called IXmlSerializable, and the attributes are [XmlRoot] and other attributes in the System.Xml.Serialization namespace.
If you want to use a relational database, SQL Server Compact Edition is free and very lightweight and based on a single file.