You will need to include the Microsoft Word Object Library in your project references.
Dim wdApp as Word.Application
Set wdApp = New Word.Application
wdApp.Visible = True
wdApp.WindowState = wdWindowStateMaximize
wdApp.Documents.Open FileName:="C:\Temp\MyDoc.doc", ConfirmConversions:= _
False, ReadOnly:=False, AddToRecentFiles:=False, PasswordDocument:="", _
PasswordTemplate:="", Revert:=False, WritePasswordDocument:="", _
WritePasswordTemplate:="", Format:=wdOpenFormatAuto