This is a reply by Brian Tilman to a similar problem with 2007. I anticipate it will work with 2010 as well:
Open Windows Explorer and click Tools>Folder Options>File Types. Locate the XLSX file type and select it. Click Advanced. In the Actions pane, select Open and click Edit. Record what you see in the "Application used to perform action" and "DDE Message"
fields because you'll need it if you ever want to restore the current behavior.
Remove the "/dde" from the end of the string in the "Application used..." field and insert "%1", being sure to include the quotes as well. Uncheck the "Use DDE" box. Click OK. Uncheck "Browse in same window" if you want Internet Explorer to open a spreadsheet
in an Excel window instead of a browser window. Click OK, then OK. Double-clicking a worksheet should now create a separate Excel window.
Brian Tillman [MVP-Outlook]