I had the same problem but I fixed it.
You should install vista from windows xp and select new installation and install it on (for example) on drive E.
I have the same drive letters on both system xp and vista.
You should not install vista from boot cd or dvd.
If you do that you should format that vista drive and reinstall windows xp and then install vista form xp. Enjoy. bye.
You can keep vista on any drive you want.
I had the same problem but I fixed it.
You should install vista from windows xp and select new installation and install it on (for example) on drive E.
I have the same drive letters on both system xp and vista.
You should not install vista from boot cd or dvd.
If you do that you should format that vista drive and reinstall windows xp and then install vista form xp. Enjoy. bye.