Install XP and linux on two separate partitions ... or even better, on two separate hard disks if you have more than one.
During the installation of Linux, you will be given the option to install a boot loader like LILO or GRUB. Install it.
That way, at startup, you'll be given a menu. You can choose to boot into Linux or into XP each time.
a boot loader DOES work, however it can mess up your MBR a lot sometimes. I would say for casual linux use, just creat a boot disk and skip the boat loader. I say if you do use a boot loader, use grub, because it's not as binded to linux as lilo is.