Excel starts workbooks that are in your XLStart folder (location varies with
versions of windows and excel, but you can use Windows Start Button|find to
search for XLStart).
Excel also starts workbooks/addins that are checked inside Tools|Addins.
If you're getting an error like:
Compile error in hidden module: AutoexecNew
Then take a look here:
http://support.microsoft.com/default.aspx?scid=kb;EN-US;q307410
OFF: "Compile Error in Hidden Module" Error Message When You Start
Word or Excel
It points at PDFMaker.xla as being the culprit.
And if you're thinking you're getting errors from Winfax, you may want to look
here:
http://support.microsoft.com/default.aspx?scid=kb;EN-US;q136874
New Workbooks Open Repeatedly Starting Excel
Excel also can open files that are referred to in the windows registry, but
usually cleaning up tools|addins or XLStart fixes most probles.
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And autoexec doesn't mean anything special to excel. Excel (still supported)
does automatically run Auto_open (located in a General module) when the workbook
opens. (As well as the workbook_open procedure that Pat described.)