Cannot open the excel files in Sharepoint 2010

Its not a issue actually its just a setting that has changed may be after Migration to 2010.

Issue : Users cannot open excel files in SharePoint 2010 from the Library and get an error

This workbook cannot be opened because it is not stored in an Excel Services Application trusted location.
To create an Excel Services Application trusted location, contact your system administrator

Normally what we tend to do is we will go to the administrator and ask him to add the library to the trusted location .

Not need . No setting to be made on the server it is just a Library setting which can be changed from the site itself . Until you want to use excel web services ( Please make a note of this )

Cause : By default the files are designed to open in browser . So it is checking for trusted location   

Resolution :  

=> Go to the Library
=> On the top click on Library and go to Library settings .
=> Under General settings you will find Advanced settings 
=> Over there you will find the option which by default is set to open in Browser 
=> you need to set it to Open in Client and there you go . 
=> Click OK and you are done .

Hope it helps and you don't disturb the administrators :)