Cannot open Excel file in C#

I have the following C# function in my project, which is supposed to open and return an existing Excel workbook object:

Application _excelApp;

// ...

private Workbook OpenXL(string path, string filename)
{
    try
    {
        if (_excelApp == null)
        {
            _excelApp = new Application();
        }

        Workbook workBook = _excelApp.Workbooks.Open(path + filename,   // Name
                                                     0,                 // Do not update links
                                                     true);             // Open read-only

        return workBook;
    }
    catch (Exception e)
    {
        _excelApp = null;
        throw new ArgumentException("Error opening " + path + filename, e);
    }
}

But when I run it with "C:\" and "scratch.xlsx", the Open() call throws the following error:

Microsoft Excel cannot access the file 'C:\scratch.xlsx'. There are several possible reasons:

• The file name or path does not exist.
• The file is being used by another program.
• The workbook you are trying to save has the same name as a currently open workbook.

The file and path does exist: I have copied the path from the error message and pasted it into a command window and the file loads up in Excel. The file is not locked: Excel can open it fine but my program cannot, even straight after a reboot. I am not trying to save it, I am trying to open it so the last option is irrelevant.

I am at a loss to understand why this simple piece of code is not working. Any suggestions would be hugely appreciated.

[edit] I have now tried opening that file from my personal network drive (M:) and from a USB stick. All to no avail.

The application is actually a Windows service, running under the local system account and generating reports. It currently write CSV reports with no access problems whatsoever. I am now trying to get it to open an excel file as a template report and fill in various fields. It is when opening the Excel file that it fails. I am thinking that the administrator account option everyone is suggesting is a red herring since it can write CSV files wityh no problem. [/edit]

--- Alistair.