I've updated my PC from Windows 7 to Windows 8..I've installed the client but its too lag..but when my computer is running with Windows 7 i didn't got any problem about the client >.<
In your case you should update your (AMD) ATI RADEON Video drivers. Click here to download your drivers. Make sure you know your Video Card model. To get your model you can use program called GPU-Z. You can download GPU-Z from here. Once you run GPU-Z you will see in the first column called Name and this is your video card model. For example my video card is ATI Radeon HD 4600 Series. Check what is your model and browse the drivers from the AMD site.