Well, boot configuration in Windows Vista can be managed using a command line tool called BCDEdit. It comes with Windows and is present in the \Windows\System32 folder. First open an Admin Commannd prompt. To do that, open startmenu, in the searchbox type cmd.exe. When it appears on the list, right click on it and choose 'Run as Administrator', you'll get a UAC prompt if its enabled. Press Continue in it.
In the Admin Console, use the following command :
Code:
bcdedit /set {bootmgr} default "{ntldr}"
This should work on a default config. I am assuming that you have Windows XP and Vista installed in dual boot config on 2 different logical drives and no other OS (linux etc) is installed.
Reboot the system and you'll get Windows XP selected by default from now on.