If you start excel from your own shortcut, try rightclicking on it and then
properties.
Check under the shortcut tab. If it looks like this:
C:\msoffice\Office10\EXCEL.EXE /e
then remove the final /e (slash e).
(The path may be different for you, though.)
If that wasn't it, maybe you have a workbook in your XLStart folder that opens
as a hidden workbook in excel.
Open excel and then check under Window (on the menubar). If Unhide is enabled,
then click it. You'll get a little dialog telling you all the names of the
hidden workbooks.
If you have a personal.xls show up in that dialog, that's usually ok. This is
where most people keep their commonly used macros/functions.
But if you see another workbook name in that dialog, that's the culprit. I'd
unhide it and see what it was.
But the next step is yours.
You'll have to determine if it's something you want to keep as a hidden workbook
or something you want to keep--but unhide
or even if it something you just want to delete
(Or keep, but move out of the way to a different folder)