Stocks Rise Slightly; AIG, Wal-Mart Up  NEW YORK (Reuters) - U.S. stocks were slightly higher on  Monday as gains in insurance company American International  Group Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=AIG.N target=/stocks/quickinfo/fullquote"&gt;AIG.N&lt;/A&gt; and retailer Wal-Mart Stores Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=WMT.N target=/stocks/quickinfo/fullquote"&gt;WMT.N&lt;/A&gt;  outweighed losses in energy stocks due to lower oil prices.