Redirect sub.domain.com to www.sub.domain.com

Hi group,

I'm sure this has been answered a gazillion times before, so I
apoligise in advance...

I have a subdomain under sub.domain.com but I'd like it to be
redirected to www.sub.domain.com instead. At the moment if I browse to
www.sub.domain.com, the server redirects to www.domain.com.

What do I need to alter to get my server to work like this? I have
root access, so I can modify httpd.conf, .htaccess or DNS records (A or
CNAME) but am confused what to change.

Thanks buckets,
Paul.