Click on start menu and click on run then type DxDiag if using XP. For Vista and Windows 7 users, they can just type DxDiag straight into the search bar from the start menu.
Click on the "Display" tab and check on the status of the following:
DirectDraw Acceleration
Direct3D Acceleration
AGP Texture Acceleration
If any of the options appear as not available, you may need to update your graphics drivers.
Pick your manufacturer from this list, and search there for the latest drivers:
Intel Download Center - Intel
NVIDIA Drivers Download - NVIDIA
AMD Graphics Drivers and Software - AMD (ATi)
If any of the options appear as not enabled, you need to goto "Display properties" and click on "Monitor" (Windows 7 users: Right click on the desktop, choose Screen resolution, then Advanced Settings, Troubleshoot and Change Settings). From there, enable hardware acceleration (or slide to full acceleration enabled).
Repeat step 1 and 2, you should be done if you get enabled across all 3. If the game still doesn't work for you follow on with the next step unless you are a Windows 7 64-bit user, in which you need to go to step 7.
Run the game as administrator. This can be done by right-clicking on the client icon and choosing it from the menu.
Run the game in 32-bit mode.