I use this function.
You'd might call it Like this:
If WorkbookIsOpen("somebook.xls") Then
Workbooks("somebook.xls").Activate
Else
Workbooks.Open FileName:="c:/My Documents/somebook.xls", _
ReadOnly:=False, Notify:=False
End If
Code:
'
Function WorkbookIsOpen(wbname) As Boolean
' Returns TRUE if the workbook is open
Dim x As Workbook
On Error Resume Next
Set x = Workbooks(wbname)
If Err = 0 Then WorkbookIsOpen = True _
Else WorkbookIsOpen = False
End Function