Object Reference Error Opening Excel Worksheet

[VBProgrammer]

I get the following error when I do an Excel.Workbooks.Open. This error occurs on some PCs but not others. The error and associated code are below.

Thanks in advanc

Error
System.NullReferenceException: Object reference not set to an instance of an object
at Excel.Workbooks.Open(String Filename, Object UpdateLinks, Object ReadOnly, Object Format, Object Password, Object WriteResPassword, Object IgnoreReadOnlyRecommended, Object Origin, Object Delimiter, Object Editable, Object Notify, Object Converter, Object AddToMru, Object Local, Object CorruptLoad

Code
Dim excelApp As New Excel.Application(
Dim excelBook As Excel.Workboo
Dim excelWorksheet As Excel.Workshee

excelApp.Workbooks.Open("c:\data\workbook1.xls"
excelBook = excelApp.ActiveWorkboo
excelWorksheet = excelBook.ActiveShee