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?