From Excel Help:
Excel does not save a file in template format if you type the .xlt file extension in a file name. To save a file as a template, click Save As on the File menu. In the Save as type box, click Template. In the Save in list, locate and click the location where you want to save the template. In the File name box, enter a file name, and then click Save.
To make the template available in the Templates dialog box (General Templates..., New Workbook task pane), save custom templates in the Templates folder. If you save the file to a different location, you can only open the file, not use it as a template. Save the default workbook template (Book.xlt) and the default sheet template (Sheet.xlt) in the XLStart folder or the alternate startup folder. The Templates folder is usually
C:\Documents and Settings\<user_name>\Application Data\Microsoft\Templates
and the XLStart folder is usually
C:\Program Files\Microsoft Office\Office10\XLStart