Hi,
assuming that you use the MFC, you could simply catch the EN_CHANGE notification from your edit control (using the class wizard), which contains code like this:
CString strTest;
m_edtInput.GetWindowText(strTest); // or use a string-member
m_btnSomeButton.EnableWindow(strTest.GetLength() != 0);
This should do the trick - it enables the button as soon as some text is typed into the edit.
Hope this helps,
Martin