The error message, alone, won't give us enough information to help you, I fear.
Could you show us the code involved that triggers the message, please? Not just the line with Workbooks.Open, but everything to do with instantiating the Excel application.
Also, have you checked that Workbooks.Open is causing the error, or could it be some other line of code?
Is your code running in some kind of loop?
What kind of project is this (WinForms, Console App, etc.)?