Now there are many possible causes for this:

1: If vista is booting (my experience of the problem) it usually indicates you have to 'tell' vista that it has to ask what to boot. (and vice versa) I think you have to use msconfig but not too sure.

2: Try a program called BOOTUS and install a boot manager on primary while registering the xp install.

And if I got these two suggestions wrong dont yell, Im only 13.