The game client can be downloaded here, just select a mirror:

Mirror 1 (FileFront)

For help with installation, please check out the FAQ.

If you already have .NET 2.0 SP1, XNA, VC++ runtime and DirectX 2008 installed on your computer, you can download this lite version of the client here (the installer does not have the 4 dependencies). If you are unsure what to download, download from the first link, since it contains all frameworks.

Here are the system requirements

A. Minimum system: CPU: Intel Pentium 4 1.0GHz or AMD Athlon XP 1000+. Graphicscard: ATI radeon 9600 or Nvidia GeForce 5600FX. 56k Modem

Recommended system: CPU: Intel Pentium 4 2.0GHz or AMD Athlon XP 2000+. Graphicscard: ATI radeon x800 or Nvidia GeForce 6800. 128k Cable

You will also need the Microsoft .NET framework, Microsoft XNA framework, Visual C++ runtime and the latest version of DirectX. All four things are supplied in the client installer.