sp_addlinkedserver [ @server = ] 'server' 
    [ , [ @srvproduct = ] 'product_name' ] 
    [ , [ @provider = ] 'provider_name' ] 
    [ , [ @datasrc = ] 'data_source' ] 
    [ , [ @location = ] 'location' ] 
    [ , [ @provstr = ] 'provider_string' ] 
    [ , [ @catalog = ] 'catalog' ]

EXEC sp_addlinkedserver 'EXCEL2',
	'Excel',
	'Microsoft.Jet.OLEDB.4.0',
	'c:\temp\authors.xls',
	NULL,
	'Excel 8.0;IMEX=1;',
	NULL

EXEC sp_addlinkedsrvlogin 'EXCEL2', 'false', NULL, NULL, NULL

To confirm:
     exec sp_linkedservers