Depending on the chipset, you may have to decrease FSB speed as well - downclocking the CPU by 25%. Some chipsets just won't let you run RAM slower than FSB, some give you a huge speed penalty.
Just give it a try, run e.g.
CTCM with PC133, then try again with PC100.
You might even get away with overclocking the PC100 sticks to PC133 - but
test well before actually using it.
Another option would be to get some PC133 used - 256 MB sticks should be very reasonably priced.
