I also found out that there are differences. And chkdsk from Windows 7 on a XP partition will convert it.
I used a Windows 7 repair CD to run chkdsk on my XP partition. It did find additional errors. But it converted the NTFS PBR - partition boot sector to the new version. In Testdisk you can compare (dump) the backup PBR to the active PBR and in plain text was bootmgr (for Vista/7) on current and ntldr (for my XP) on backup. I had to restore backup.
Or you can use the "bootsect.exe /nt52 c:" command to restore an XP PBR.