Question details: How to uninstall Linux? Can Linux modify the MBR?
Linux uses different kind of partitions different from the ones used by Windows XP. Windows XP uses Fat32 or NTFS and Linux uses EXT partitions.
If you install linux on your computer a full XP format wouldn't delete the EXT partition. You should use a program like Partition Magic to recover your EXT partition and transform it in NTFS.
If you want to uninstall Linux insert the Linux Cd in the drive, boot from it and choose the option to uninstall.
Linux in some cases will rewrite your MBR with it's own software(Lilo or Grub)