Here's how to do it. start up xp log in etc.
start, right click on my computer, Manage, Disk management.
this will get you to the partition window. shrink a partition (right click shrink and follow the steps) so you have however much space you want for the linux installation. shut down the pc and restart with the linux disk inside. follow the install steps the when it asks you where to install press manual. in the empty hard disk space create a 1gb swap partition and a make the rest ext3. label this volume "/".
it will then install into the partition and most likely, depending on the distro you will be given an option at startup, wheather to boot between xp and the distro.
Dual booting vista and ubuntu