This error typically occurs as a result of the software update on your computer. YAPI displays the following OMG! Error in YAPI message when you open existing Form or are trying to create a  new one.

******** ERROR ******** System.IO.FileNotFoundException: Could not load file or assembly ‘Microsoft.mshtml, Version=7.0.3300.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a’ or one of its dependencies. The system cannot find the file specified. File name: ‘Microsoft.mshtml, Version=7.0.3300.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a’
at Neva.HTMLEditorNew.HTMLEditorNew_Load(Object sender, EventArgs e)
at System.EventHandler.Invoke(Object sender, EventArgs e)
at System.Windows.Forms.Form.OnLoad(EventArgs e)
at System.Windows.Forms.Form.OnCreateControl()
at System.Windows.Forms.Control.CreateControl(Boolean fIgnoreVisible)
at System.Windows.Forms.Control.CreateControl()
at System.Windows.Forms.Control.WmShowWindow(Message& m)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.ScrollableControl.WndProc(Message& m)
at System.Windows.Forms.ContainerControl.WndProc(Message& m)
at System.Windows.Forms.Form.WmShowWindow(Message& m)
at System.Windows.Forms.Form.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at A.L.QJ(Message& A)
at A.NB.QJ(Message& A)
at A.TH.IR(Message& A)
at A.TH.QJ(Message& A)
at A.L.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)


 To resolve you need to run vs90_piaredist.exe file found in PIA folder inside YAPI folder. To find this file, right-click YAPI shortcut on the desktop, and select Properties option.

On YAPI Properties window click Open File Location button.

Double-click to open PIA folder 

Double click vs90_piaredist.exe file to run.

Restart YAPI.