You can try as follows,

1. Keep the job owner as sa (sysadmin) and in the job step use xp_cmdshell

2. Ensure that the Sql Service account has necessary permissions to the folders you wish to access because xp_cmdshell for a sysadmin will run under the context of Sql service account.

Also please check if this link helps, http://support.microsoft.com/kb/241643/[/url]