[Pierre-Marie Baty]

Frosty is right, the partitions' bootloaders can only be executed if the master boot record "chainloads" them (which is what Grub does). A hard disk is sliced in the following way:

512 bytes => MBR (+partition table)
512 bytes => partition 1 boot record
all the rest => actual partition 1
512 bytes => partition 2 boot record
all the rest => actual partition 2
etc.

Where Whistler is wrong though, is that everytime Windows will feel like crushing the MBR again (which it does quite often), any custom bootloader you have will zap. You will still need a UNIX recovery disk to fix it manually, and it's not necessarily trivial.