How can I log in to my Guild Wars Account automatically when I launch the Game Client?
You have to add some command line arguments to Gw.exe when you start the game up.
The two that you need to add are
From the link:
Create a shortcut to the gw.exe or edit its properties and add following options after “C:\Program Files\Guild Wars\gw.exe” -email mail(at)provider.end -character "Name Of Your Charakter" -password qwerasdf
Be aware: Your password is saved in cleartext!
Other command line options can be found on GuildWiki