Are you doubleclicking on a .xls or a .xlt file?

If it's a .xlt file (a template), then this is the behavior you want.  The
purpose of a template is so you can create a new workbook based on that
template.

If it's a .xls file...

The first thing that I'd try is to reregister excel to set it back to factory
defaults.

Close Excel and
Windows Start Button|Run
excel /unregserver
then
Windows Start Button|Run
excel /regserver

Then test it out.

If that doesn't work...

Close excel
start windows explorer
tools|folder options|file types tab
scroll down to the xls extension (microsoft excel worksheet)
click the advanced button
click on the Open item in the Actions box
click set as default
(ok your way out)

If the default action was set to New (bolded indicates default), then that could
have been your problem.