getting native error 18456 Error = login failed for user(username) when using BCP

Hi ,
can any one help in exporting data to a flat file by using BCP,
iam trying to use BCP but iam getting following error
Error : native error 18456 Error = login failed for user when using
BCP
thanks well in advanceexec master..xp_cmdshell 'BCP northwind..orders OUT
c:\test1.txt -c -C850 -SServer -Usa -Ppwd'
"RPK" <praveen.rampally@.gmail.com> wrote in message
news:1143093192.808073.125810@.u72g2000cwu.googlegroups.com...
> Hi ,
> can any one help in exporting data to a flat file by using BCP,
> iam trying to use BCP but iam getting following error
> Error : native error 18456 Error = login failed for user when using
> BCP
> thanks well in advance
>