There are four main causes of misaligned labels (though in your case you can probably discount the last). My guess would be '2' or '3'.
1. You have added a header/footer to the normal template (rename the normal template)
2. The page size set in the printer properties doesn't match the page size of the label sheet.
3. You have a scaling option set in the zoom section of the print dialog
4. Your printer has poor paper handling. Some printers will slip when faced with label stock and thus are incapable of feeding the label correctly. Cleaning the rollers in the print path may help.
Check by printing on cheap plain paper and holding it over the label blank. It is easier to check if you temporarily add borders to the table cells.
If that doesn't fix it, take a look at
http://www.gmayor.com/Envelope_Label_Add_In.htm