Calpine Accounts Questioned  NEW YORK (Reuters) - Calpine Corp. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=CPN.N target=/stocks/quickinfo/fullquote"&gt;CPN.N&lt;/A&gt; has been  struggling badly ever since the Enron Corp. collapse in 2001  exposed the weaknesses of its power generation and trading  business, but a recent study of its accounts suggests worse may  still be to come.