try changing

LRESULT CALLBACK CEasyWindow::ProcessMessage(HWND m_hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)

to

LRESULT CALLBACK ProcessMessage(HWND m_hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)

(i.e. make it a global function). Being a member function is adding a hidden this pointer to the arglist, so your code actually compiles to something along the lines of ....

  ProcessMessage(CEasyWindow* this, UINT msg, WPARAM wParam, LPARAM lParam)

 and therefore it doesn't match the winproc prototype.

Regards,
Meredith Shaebanyan