Ho deciso di partizionare l'HD per installare XP per far partire sto benedetto NWN2, ma dato che le cose non vanno mai lisce al primo colpo, ecco che dopo la partizione e l'installazione di XP, Vista non veniva più caricato, ma XP partiva in automatico senza fornire le opzioni per un dual Boot.

Al che Francis mi ha segnalato un utile articolo KB al riguardo, che tra i sintomi recita:

If you install an earlier version of the Windows operating system on a Windows Vista-based computer, Windows Vista no longer starts. In this case, only the earlier version of the Windows operating system starts.

Qual è la causa?
These issues occur because earlier versions of the Windows operating system are incompatible with the new Windows Vista startup method. Windows Vista uses a new Boot Configuration Database (BCD) store. This store contains a boot menu and all the information about operating systems that are installed on the computer. Therefore, a Boot.ini file that is from an earlier version of the Windows operating system cannot be used to start Windows Vista.

Come risolvere?

Note You can run the commands in the following procedure by using the command prompt. If you run these commands in Windows Vista, run them at a command prompt that has elevated user rights. To do this, click Start, click Accessories, right-click the command-prompt shortcut, and then click Run as Administrator.

1. Use Bootsect.exe to restore the Windows Vista MBR and the boot code that transfers control to the Windows Boot Manager program. To do this, type the following command at a command prompt: Drive:\boot\ Bootsect.exe –NT60 All
In this command, Drive is the drive where the Windows Vista installation media is located.

2. Use Bcdedit.exe to manually create an entry in the BCD Boot.ini file for the earlier version of the Windows operating system. To do this, type the following commands at a command prompt.

Note In these commands, Drive is the drive where Windows Vista is installed. • Drive:\Windows\system32\Bcdedit –create {ntldr} –d “Description for earlier Windows version”

Note In this command, Description for earlier Windows version can be any text that you want. For example, Description for earlier Windows version can be "Windows XP” or “Windows Server 2003." 
• Drive:\Windows\system32\Bcdedit –set {ntldr} device partition=x:

Note In this command, x: is the drive letter for the active partition. 
• Drive:\Windows\system32\Bcdedit –set {ntldr} path \ntldr
• Drive:\Windows\system32\Bcdedit –displayorder {ntldr} –addlast
 
3. Restart the computer.

Tutto l'articolo qui: Windows Vista no longer starts after you install an earlier version of the Windows operating system in a dual-boot configuration