How to choose .NET version to compile project?

You can't I'm afraid. VS2005 only works with .NET framework 2.0. You'll need the appropriate versions of Visual Studio to work with other versions:

• 1.0: Visual Studio .NET
• 1.1: Visual Studio .NET 2003
• 2.0: Visual Studio 2005 / 2008
• 3.0 / 3.5: Visual Studio 2008
• 4.0: Visual Studio 2010

jmservera points out that some 3.0 libraries are compatible with Visual Studio 2005. For example WCF is compatible, while LINQ isn't. Some of these require additional downloads, for example to use Windows Workflow Foundation comfortably in Visual Studio 2005, you need to install this extension.

This is just for your information: If you are using VS version greater than or eqaul to 2008, you can do this by right clicking the project you want to build, click Properties, and select the Application tab (if not already selected). Change the value in the Target Framework dropdown list.

_{(source: scottgu.com)}