Nortel Warns of Weaker Sales  OTTAWA (Reuters) - Nortel Networks Corp. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=NT.TO target=/stocks/quickinfo/fullquote"&gt;NT.TO&lt;/A&gt; on  Thursday warned that third-quarter sales will fall below its  second quarter and that full-year 2004 revenues at North  America's largest telecom equipment maker will lag the overall  sector.