How to know which Operating System is running ?

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C#

.NET

Hi Every one....
i want to execute disk fragment form my coding by calling windows API
and my code is

Process.Start("dfrg.msc"); //It is for Windows Xp

,but i have faced a problem that for different operating system like Windows 7 or Vista the command code is different.
Means for Windows 7 the code is

Process.Start("dfrgui");

so,how can i know which operating system is running in computer ??

i just follow this code

MessageBox.Show(System.Environment.OSVersion.ToString());

and for XP service pack 3 i get this message

Microsoft Windows NT 5.1.2600 Service Pack 3

Is there any code to detect the exact OS so that i can run the Process by If else clause .