如何检查.Net Framework是否已安装[C＃/ WPF]

Before starting my application (on Form Load) I want to check and ensure that the .NET framework (3.5 or higher) are installed on the machine - if not I want to prompt the user to download and install it (simple MessageBox with a INTERNET LINK to the Microsoft WebPage would be the best idea no?) So, this actually entails two questions...Before starting my application (on Form Load) I