This should probably have been posted to the Windows forum.
Derive a new control from CEdit and call it CAlphaEdit. Overload the OnChar() method and check if characters entered were non-char, if so, block them.
If you need anything else, or don't understand this example, post back.
class CAlphaEdit : public CEdit
afx_msg void OnChar(UINT, UINT, UINT);
void CAlphaEdit::OnChar(UINT nChar, UINT nRepCount, UINT nFlags)
CEdit::OnChar(nChar, nRepCount, nFlags);