What distro? And what exactly is the file you are trying edit?
I've distro hopped a number of distros, and not yet run into a file named "xstartup." xorg.conf, but not "xstartup."
The more precise and detailed information you provide, the more likely someone can help you.
Also, generally, you must edit configuration files in /etc with root privileges on the local box. Often, user can open the files, but cannot save changes to them.
Last edited by frankbell; 01-29-2011 at 10:30 PM.