Excel opens 2 instances unexpectedly

Hi,

I have Office 2013 H&B installed on a Win7 Pro x64 machine, and when I open an Excel workbook (from say, the desktop) it spawns two instances of Excel, one blank (no opened workbooks) and one instance with the workbook I intended to open.

If I just open Excel.exe one instance opens as expected; the issue only occurs when I open an already saved workbook.

Any ideas anyone?

Thanks