Excel Services Refresh Fails

1. check the sharepoint log (C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\LOGS if you get the error Excel Services can not use credential delegation when it is configured to use a trusted subsystem with Windows SharePoint Server
2. change Excel Services to use Delegation Mode

stsadm -o set-ecssecurity -accessmodel delegation -ssp SharedServices1

stsadm -o execadmsvcjobs

iisreset