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It was during a certain time only... it was only playable during the E3.
Keep your eyes peeled though, you never know what might happen again in the future :>
From the Guildwars FAQ:
I obtained the game client, but cannot log in?
The game client in question was provided as part of our E3 for Everyone event, which took place from May 10 through May 14, 2004. That event is over. We may have other pre-release events in the future, at which time you can use the game client that you have, or obtain one at the time of the event