Open spreadsheet sometimes opens new copy of Excel - WHY?

Hi
Sometimes when I open a spreadsheet from recent document list or a pinned document (right click icon on taskbar) Excel opens a new copy of the programme, which then means that I can't ctrl-tab between the documents.
It never used to do this. Why is it doing it now? Are there settings I can change? This is REALLY annoying.
If I go ctrl-O to open document it always opens in current Excel programme.
Excel 2010 student, Win 7 64-bit Pro, 8GB RAM
Thanks in advance

It is same as Excel 2007 and Pc confg is not considering issue

And that means...?
Sorry to be dim, but that makes me no clearer as to why this is happening now & didn't used to