The PC 2100 RAM is not as fast as the PC3200. You might have better luck if you slow the PC3200 down to PC2100 speeds in the BIOS. Or it may be that your motherboard won't support the PC2100 ram.
can you tell me how to do that?
If your computer is a pre-built (HP, Dell, Gateway, Cmpaq etc.) probably not as they lock their BIOS's. If you built it, I suggest you look in your motherboard manual for info on updating your particular BIOS.