xp_cmdshell access denied output

I am trying to execute a batch file using xp_cmdshell: EXEC
xp_cmdshell 'c:\testing\runbattest.bat'

When I do this, I get an output that says "Access is denied."

Here are the contents of my batch file. dir c:\windows\*.exe > c:
\test.txt

The batch file works fine, but I can not run it from sql. It should
be noted that I am using SQL Server Express. Do I need the full blown
version?

Thanks,

Matt