For the sake of my privacy I'd like to boot my computer or windows only with the help of an external storage media.
Is this even possible? If so, then how?
Assuming DMA57361's assessment of your needs in the comments are correct, BitLocker will let you do full-drive encryption with decryption credentials stored on USB key. There are other solutions as well (TrueCrypt I think has the same functionality) but I suggest BitLocker as being baked into Windows already. ;)
I am not aware of any tools that simply lock the bootloader to a key without actually encrypting the drive. (If you're concerned about your data, you'd have to encrypt the drive anyway to keep people from just booting to a LiveCD or even just taking the drive.)
EDIT: To enable BitLocker, you need to be running Vista or 7 Ultimate for BitLocker. It's pretty straightforward - put 'BitLocker' in the Start Menu, find the drive in question and enable BitLocker and follow the steps.
Enter BIOS, set boot devices as USB. If you have an option for second & third boot devices - keep it as network or none.