You probably face to this issue when you install your MOSS2007 into Windows 2008 Server.
Not very sure why this issue pop up when you start your excel calculation services from your SharePoint Central Admin…, but you can solve it with simple command here.

stsadm -o provisionservice -action start -servicetype "Microsoft.Office.Excel.Server.ExcelServerSharedWebService, Microsoft.Office.Excel.Server, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c"