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I want to install different version of Windows on a PC.

  1. I installed Windows XP on a first harddrive.
  2. I add a harddrive (HD) on my PC, split it in three part
  3. Since I have only one SATA cable, I unplugged the first HD to plug the DVD drive.
  4. I installed a working version of Windows Seven on the first partition of the second HD.

I plugged back the two HDs but I have no dual boot screen on startup : Windows XP boots automatically.

Additional information: Both system are installed on the same C: drive letter since there is no option during Windows 7 installation processus.

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Is the BIOS pointing at the right hard drive? – Dave Mar 12 '13 at 13:48
Boot from the Win7 DVD and use bootrec /RebuildBcd as detailed here. – Karan Mar 13 '13 at 1:29

OSes don't know of each other, that's why you don't have boot menu.

Configure your BIOS to boot from the Win7 hard drive. I won't give you details as it differs between motherboard models.

Then install EasyBCD and use it to add Windows XP to the bootloader menu.

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seeing my additional information, will-it work if the two system considered themself installed on letter C: ? – Martin Delille Mar 12 '13 at 17:27
It will work, but you'll have different letters for the same drive (both systems will call their own drive C:) – gronostaj Mar 12 '13 at 17:53

Since you installed Win XP on one HDD, removed the HDD, installed Win 7 on the other HDD, win 7 does not know about Win XP being installed, similarly XP does not know about 7

You can switch between them for now by changing the primary boot device while booting, but for a long term fix, you will need to play around with your bootloader settings to "tell" win 7 about XP, and then boot via 7, OR reinstall 7 on the same HDD as XP

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