Do I need to format both partitions, when downgrading from Windows 7 to XP?

Currently have 2 partitions, both are in NTFS. One for OS, and another for data. I've heard that XP & 7 uses different NTFS versions, so it's good idea just format first partition when installing XP and my data on second one will left untouched? Or just copy all data to external HDD and format all partitions with XP retail CD?