You probably used the new (like Word 2007) Equation Tools. Then the conversion is simple. The old equation editor is still available, but you need to take extra steps to use it (Insert -> Object -> Microsoft Equation 3.0), so you probably didn't use it.
When you copy an equation created using Equation Tools and paste it into an editor, such as Notepad, it should be automatically transferred in MathML format.
If this does not happen (and instead you get only some characters in the equation), check the parameters of the equation. You can open them on the tab "Equations Tools", "Design"; click on the very small icon in the lower right corner of the Tools group:

" " ; " MathML ".