"DirectX is a collection of application programming interfaces for handling tasks related to multimedia, especially game programming and video."
You can read more here: http://sites.amd.com/us/game/technology/Pages/directx-11.aspx
The latest games require the latest DirectX
So getting the latest DirectX drivers is necessary.
And no , Xp dosnt support DirectX 11, it only Supports DirectX 9.0
I would recommend to get Windows 7 32-bit. ( 32-bit because you have only 2gb ram)
Get DirectX 11 ( its free)
And update your drivers too.
That might solve the problem.