In VISTA when you try to open a word document or excel sheet from your program [VB.NET code], it  always gets minimized to the taskbar.To resolve this issue you can use the following command of AppActivate.

[VB.NET]

' CREATE AN OBJECT FOR WORD
Dim objWord As Object
objWord = CreateObject("Word.Application")
objWord.visible = True
' OPEN THE WORD FILE
objWord.Documents.Open(filename:=FULL PATH OF YOUR FILE)
' SET THE FOCUS TO WORD
AppActivate(objWord.Windows(1).Caption)

[VB.NET]

Similarly for Excel

[VB.NET]

' CREATE AN OBJECT FOR EXCEL
Dim objExcel As Object
objExcel = CreateObject("Excel.Application")
' OPEN THE EXCEL FILE
objExcel.Workbooks.Open(filename:=FULL PATH OF YOUR FILE)
' SET THE FOCUS TO EXCEL SHEET
AppActivate(objExcel.caption)

[VB.NET]