How to make window clear when scrolling

I derived a CWnd class named C2WinBox and override OnVScroll function
With SetScrollPos, ScrollWindow, and UpdateWindow functions.
Then I used C2WinBox object created a Window. The Window has WS_VSCROLL style.
Then I created some controls on The window such as CEdit box, CCombobox, CStatics etc.
All controls were created dynamically.
But when I Scroll the window, the contents on the window are mess up.
How to make the window clear when scrolling?
Do I work on the wrong way?
I appreciate any reply.