You'll need to open both .xls spreadsheets at the same time.
In the destination spreadsheet, click the cell that you want to link the data into. In this cell, type '='.
Click Window in the menu bar and select the other workbook.
Find the source cell and click it, then press 'Enter'.
Switch back to the destination workbook, and you'll see the link has been made.
As long as both workbooks are open, any changes to the source data will automatically be updated in the destination.
If the source workbook is not open, you'll be prompted when you open the destination workbook to update the data from the external workbook.
Alternatively, you can force a manual update. In the menu bar, click Edit, Links. The Edit Links dialog box opens. The Source File list box contains a list of all linking resources used in the active workbook. Select the link reference you want to update, and click the 'Update Links' button.