There is a Windows setting for that so perhaps there might be an API call you can use. To find it in Windows yourself have a look at the mouse settings in Control Panel. There should be a tick box labelled "Automatically move pointer to the default button in a dialog box" or something similar.
On a related note, you can control which button is the default on a MsgBox. E.g. instead of just "vbYesNo" try "vbYesNo + vbDefaultButton2" to make No the default option. This allows you to change what happens when the user presses the ENTER key, which might be a good half-way house?
PS - personally I'd avoid doing exactly what you propose. Some people like the mouse to do that, others don't. Those that do like it can tick the box in Control Panel (or you can show them how) so why interfere? You might just confuse the people you are trying to help!