Can't open already opend files since changeover.

[Phil Smith]

Before: Windows XP, Excel 2003.
When clicking on a network spreadsheet that someone else has opened,
after a second a box comes up giving you the option of opening it in
read only, or being notified when the file is closed.

Now: Windows 7-64, excel 2007.

When clicking on a network spreadsheet that someone else has opened,
after a long time, (a minute or more,) a message comes up saying that
the file cannot be opened.

How can I get the previous behavior?

Phil