Since you can't get into the BIOS (Basic Input/Output System) I think that S is System, changing the CPU speed might be a problem... but any how, to see if this works you'll have to clear the BIOS.
To do this... you'll need the manual that came with the motherboard, look through all the pictures of the motherboard (overall pictures of the motherboard and zoomed in ones) Look for something that'll say something that'll give a description of 'Reset CMOS' or 'Clear CMOS' or it might use BIOS instead of CMOS.
What you will end up doing is using a little itty bitty black jumper that goes across 2 pins.
I'm going to try to give a poor visual aid. A 'o' is a pin and a '[]' is a the jumper covering a pin. Seen down below there are 3 pins and the jumper covers 2 of them.
o[][] <----- Let's say this is how it came when you bought it.
[][]o <----- This is when you move the jumper to the other two pins to clear the CMOS/BIOS
So after you do that, I would recommend pulling the battery out : (looks like an oversize watch battery) and unplug the power supply, let it sit for 30 minutes to 60 minutes to ensure that it gets cleared.
Put everything back in it's place including the jumper and battery. Turn it on and when it POST (where it shows RAM info. CPU info. detecting hard drives, etc.) Press the 'delete' key. (delete key is the usual for getting into the CMOS/BIOS, it maybe different for that motherboard).
If you can't find the jumper to clear the CMOS/BIOS, just skip that and do the battery and power supply.
If you do happen to get in the CMOS/BIOS look around in there for your CPU speed and set it to a lower speed that what you have.
Another way is to change the CPU speed is to move switches (if any are on the motherboard). But let's see if the above works or not.