Vista's file system.

Previously I have installed Vista on my computer which has 2 NTFS partition. I installed Vista on the first partition and I didn't touched the 2nd partition because it has data on it. But when I install a program the Vista shows some errors. So I tried to troubleshoot the problem but I end up reinstalling XP again. When I install XP it can't detect the first partition where I installed Vista. So I end up re-partitioning my hard drive again, thus the 2nd partition was deleted.

What is the file system of Vista that it can't be identify by XP?

I hope somebody can enlighten me?

Thanks,