Read my response at this discussion web page.  they had problems with .pdf.  
You may be havving the same problem with .xls.  Go to xls instead of pdf in
my response.  There is a %1 parameter that may be missing in your window
explorer when opening EXCEL files.

http://www.microsoft.com/office/community/en-us/default.mspx?dg=microsoft.public
.excel.worksheet.functions&mid=a64356f6-0a46-47d2-afde-5848a1fe8c74

Sometimes one of these works when you're having trouble with double clicking on
the file in windows explorer:

Tools|Options|General|Ignore other applications (uncheck it)

--- or ---

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

The /unregserver & /regserver stuff resets the windows registry to excel's
factory defaults.

Maybe one of these will work for you.

when excel is opened (a workbook must also be open), then go to
Tools=>Options and see if Ignore Other Applications is checked.  If so, then
uncheck it.

A second cause of the problem can be solved by closing excel, then in the
windows start menu, select run and do

excel.exe /regserver

and click OK.  This will reregister Excel's default settings.