As for the PC133 memory running at 133MHz...it all depends on you CPU. If your Athlon/duron can run at a maximum of 200MHZ, it will either run at 200 or 100MHz depending on whether you have DDR ram or not. some Athlons have a maximum fsb of 266MHz, thus it can run at either 266 or 133MHz. In your case...it sounds like you have a athlon with a maximum fsb of 200 and only SDRAM...thus will only run at 100MHz
If you have been messing with your mobo recently to make it stop working...then you have changed a switch that is stopping it from working. I did it one time and had to get a pro to sort it out for me.