Hi,
Are you sure XP server is starting up correctly ?
Have a look at the XP Server error log at $SYBASE/install/<XP Server Name>.log to ensure XP server is starting up correctly when you try to
run xp_cmdshell. Also grep for xpserver process to check it is now running.
There is a good Sybase faq on XP server at:
http://www.sybase.com/detail?id=20200
Regards
Declan