How can i check My DirectX version??

I have used Windows 98 and Windows Me but never found the tools that give me the access to check the DirectX version.
Now I am using Windows XP and still i don't know where to find that tool to check my DirectX version.
And I intend to use Windows Seven in future.
So I want to know How can I check My DirectX version in windows XP.
At first answer me this question and then I will like to know about the same thing how to do in Windows seven.
For more help I am now using windows XP professional and service pack 2.
I hope this may be helpful for you to answer.
And if you can please tell me what are the basic differences between DirectX 9 and DirectX 10.
Why those upgraded version of DirectX is supplied.
Is DirectX is too important to play games in Windows operating system.
