Can you post the contents of your ..\MSSQL\LOG\SQLAGENT.OUT file too please.

From SQL Server BOL "The Windows process spawned by xp_cmdshell has the same security rights as the SQL Server service account."

So make sure the account that starts up SQL Server...

Sounds to me like SQL Server is performing a CHECKPOINT (flushing dirty pages to Disk) when you get your disk queue spike.. You can use Perfmon to verify by...