TPM stands for Trusted Platform Module, which is a chip to be included in the PC for extra security of the system. It works hand-in-hand with the BitLocker function in the Windows Vista to encrypt the entire PC system when necessary. It also provides other security functions such to prevent system configuration to be changed by un-authorized people. If you PC is used by many people, or if you PC is a portable device that may potentially lost, I strongly suggest you to get the TPM. If it is only a desktop PC only for yourself, then TPM is nice to have but not necessary.