prova così

codice:
[boot loader] 
timeout=30 
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS 
[operating systems] 
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect 
multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="Windows Server 2003" /fastdetect /NoExecute=OptIn
oppure (se i due dischi sono fisicamente diversi )

codice:
[boot loader] 
timeout=30 
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS 
[operating systems] 
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetect 
multi(0)disk(0)rdisk(1)partition(1)\WINDOWS="Windows Server 2003" /fastdetect /NoExecute=OptIn