Access denied is typically because the account in the servicesndoesnt have access.... examples are when you put files in c:\users\specificperson\desktop or my documents and the account cannot access. Move the files to a non private directory would fix it.

If the account does have access then itnmight be because thenfile is open in excel at the same time this process isntrying to open it as well.

If you give the exact error and path to files we could help further.