Nortel Warns of Weaker Sales  OTTAWA (Reuters) - Retreating from an upbeat forecast it  made just last month, 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 2004 revenue growth at North America's largest  telecom equipment maker will lag the sector.