Due to some bad luck with installing soundcard drivers I've decided to get a new harddrive and install XP (Before anyone thinks im pirating my windows licenses; I get them from Microsoft Academic Alliance) on it.

I currently have 1 HDD only, with Vista x64 installed.

The last time I tried this, XP booted straight away and suddently my computer had forgotten that I had Vista installed on another drive. What should I do to prevent this from happening again?
And what can I do to fix it if it the same thing does happen again?