It depends which version of Visual Studio:
- In 2002, all projects use .Net 1.0
- In 2003, all projects use .Net 1.1
- In 2005, all projects use .Net 2.0
- In 2008, projects use .Net 2.0, 3.0, or 3.5; you can change the version in Project Properties
- In 2010, projects use .Net 2.0, 3.0, 3.5, or 4.0; you can change the version in Project Properties
- In 2012, projects use .Net 2.0, 3.0, 3.5, 4.0 or 4.5; you can change the version in Project Properties
Newer versions of Visual Studio support many versions of the .Net framework; check your project type and properties.