While you cannot have multiple windows open for chat, you can filter both who you are talking to, and who you are viewing.
Use the checkboxes at the top of the open chat window to control which "channels" you are veiwing. Uncheck any that you do not wish to see.
The tabs above the textbox you type in, will control who you speak to. If you wish to only speak to your guild, use the Guild tab, or start your text with @. (Each tab has a special character you use, to automatically send that text to the proper channel, without having to click that tab.)
The different channels are color coded, so you should have no trouble distinguishing different channels even if you are viewing all of them at once.
Also, you can use the Customize button on the main menu, to customize the size and position of nearly every element of the Guild Wars interface. You could, for instance, increase how tall your chat window is, so that you can view more text, etc. I encourage you to play around with this feature, as it can even help with your gameplay, keeping all those GUI interfaces up so you can see them, and out of your way while playing.
