In general this would be called under-clocking I guess, that is to tweak the cpu frequency. However this is not always possible.
At bios level this is totally dependent on the motherboard model.
Some intel motherboard allows you to tweak the memory and the cpu frequency, however with some limitations though. Mostly high-end extreme boards will have this feature but not all boards.
So if you do not see any option in the bios that allow you to modify the cpu frequency or memory frequency, then it is not a high end board that will allow this feature.
Unfortunately at this level there are no application that will allow you to reduce the cpu frequency at bios level or even dos.. well .. unless if somebody know how to do it with some low level programming. . I am not at this level yet... If there is any body out-there who can advise you of a better solution, i would be happy to know as well.
Hope this will be useful.
There's software programs that you can use to slow the computer down like, Mo'Slo. There is a program called Dosbox as well.