IBM Retakes Top Supercomputer Rank as NEC Slips  NEW YORK (Reuters) - IBM &lt;A HREF="http://www.reuters.co.uk/financeQuoteLookup.jhtml?ticker=IBM.N qtype=sym infotype=info qcat=news"&gt;IBM.N&lt;/A&gt;, the world's biggest  computer maker, has regained its position at the head of the  Top 500 list of supercomputer installations, dislodging rival  NEC Corp. &lt;A HREF="http://www.reuters.co.uk/financeQuoteLookup.jhtml?ticker=6701.T qtype=sym infotype=info qcat=news"&gt;6701.T&lt;/A&gt; of Japan, the company said on Monday.