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; warned on  Thursday that third-quarter sales will fall below  second-quarter levels and that its 2004 revenue growth rate  will lag that of the telecoms-equipment sector as a whole.