Using the ShellExecute api function, it should change the already opened IE's URL address and if no IE is open, than it will launch IE and also navigate to the site.
Code:
Declare Function ShellExecute Lib "shell32.dll" _
Alias "ShellExecuteA" (ByVal hwnd As Long, ByVal _
lpOperation As String, ByVal lpFile As String, ByVal _
lpParameters As String, ByVal lpDirectory As String, ByVal _
nShowCmd As Long) As Long
Public Const SW_SHOWNORMAL = 1
ShellExecute Me.hwnd, vbNullString, "http://www.vb-world.net", vbNullString, "c:\", SW_SHOWNORMAL