Athlon 1.3 (266FSB) on a MSI KT4AV-L, PC act slow

Hi,

I have the MSI KT4AV-L with AMD Althorn 1.3 (266FSB) and 512MB DDR 266 Memory. Before I updated to this motherboard, I was using the ASUS A7V266 motherboard. With the ASUS motherboard and the AMD Althorn 1.3 and 512MB DDR 266 Memory, My PC was working perfectly fine and fast. Since I updated to MSI motherboard, My PC boot up very slow, and sometime; it will not be able to detect one of my hard drive or one of my CDROM. And more importantly, it takes a long time to detect the IDE Drives at boot up. And once it gets to the Windows XP, My PC acting very slowly. Do you know why?
And can you tell me the correct configuration for the BIOS (CMOS) in detail? It just doesn’t feel like it is working at the full speed.
Oh, one more detail, I set it to be 133MHz, and the startup screen is showing me that I have a 1350MHz CPU, instead of a 1333MHz. Could it be a Memory problem? I set the memory to 266MHz!
HELP please!!!
Thanks