A family of Microsoft relational database management systems designed for ease of use.
I am perplexed because I use similar code and never had such problems.
Here are a couple things to try/check.
What happens when you open the Workbook manually? Is it in read-only as well?
Set wkb = xlApp.Workbooks.Open("C:\myFile.xls")
FYI: Opening an *.xls in Excel 2007 should have no impact on being able to edit and save changes to the workbook.