Voici la procédure avec rEFit (en anglais, si tu ne comprends pas dit le moi je te traduirais) :
Simply download rEFIt:
And run the installer when the DMG mounts. An "efi" folder will appear inside Macintosh HD. Leave it. Make a blank partition for Windows (if you haven't already). Any size will do. Insert the Windows Install DVD/CD into the external drive.
Reboot...
Hold Option...
Select the disk with the name "rEFIt" in bold. After a short delay, a logo will appear "rEFIt". And a series of icons. Press the corresponding arrow key to go to the disc. Select it, and it will then boot into the Install CD**.
** Denotes that this process DOES take a lot of time, the computer or rEFIt hasn't crashed, just wait it out, everything can be sluggish.
Make sure the Windows partition is formatted as FAT (MS-DOS). If you made it with Boot Camp Assistant, it will be. Otherwise, open up Disk Utility and do it.
Once in the Windows installer:
For Vista, format the disk to NTFS in Drive Options directly from the Installer. If it says "Windows cannot be installed onto this Disk. The selected disk is of the GPT Partition Table", go back to OS X and format the Partition to FAT and select the appropriate partition for install when you are back in.
For XP or previous, simply continue the Install, For gods sake, make sure you look at which partition you install to , I didn't once, and lost everything.
Above is exactly what I did. I wrote it all down so I wouldn't forget it. I knew it may come in handy