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Yesterday evening as I was about to go to bed one of my developers Doug, sent through a link to a forum post outlining a serious calculation error in Excel 2007.

This afternoon David Gainer, in the Excel team, posted an official response to the Excel 2007 Calculation Error.
He outlines the background to the problem, the specific issue in Excel 2007 and the solution.

It looks like it will be 10-15 days before a patch is distributed.

I will keep you all updated.

Navipane contacted me to review their addin for Excel on this site.
I went to the Navipane website, downloaded, installed and used the Navipane addin for Excel. I noticed there a bunch of similar addins for other Microsoft Products, notably PowerPoint, Word and Access.

I am using Excel 2007 so this review relates to how I interacted with Navipane in that environment.

When installing you are required to have Excel closed.
The first thing I noticed after opening Excel again was that there was an addin menu called Navipane in the ribbon, and a resize able tabbed window had opened up on the right hand side of the Excel pane titled Navipane System.
The tabs listed are Home, Sheets, Favorites and History.

Home

From Charley Kyd's Excel Newsletter.

When readers ask questions that I think others would find useful, I try to take the time to frame my answer as an article.
Recently a reader asked about the XNPV and XIRR functions, which are new with Excel 2007.
These are really useful functions. Suppose you have a column of cash flows and a column of dates. The dates don't need to be periodic. These functions find the Net Present Value and the Internal Rate of Return for that schedule of cash flows.
My reader wondered if there's a way to do the same thing for Future Values.
It turns out that the SUMPRODUCT function can do exactly what he needed. Better yet, this function allows Excel 2003 users to calculate both the future and present values of the same cash flow schedule.
See Find Future & Present Values From Scheduled Cash Flows In Excel 2003 and Excel 2007.