Think you got the wrong idea of installing a Linux.
You should have free unallocated hard disk space to create the partitions first, one for Ubuntu and one for swap of sizes 5 and 0.5Gb respectively, and tell the installer to install Ubuntu into it. These two partitions can be created using the Ubuntu CD and the partitioning tool is cfdisk.
When you install Ubuntu its boot loader will take over the hard disk MBR to dual boot the two systems.
Any time you want to ditch Ubuntu you boot up the Ubuntu CD using the same cfdisk to remove it or Winxp's disk management program which can remove Linux partitions but not creating them.
The Linux boot loader cannot be removed but can be overwrite with XP's MBR. You do it by booting XP installation CD, go into recovery console and type
Thereafter you can download "gparted" or "Parted Magic" to resize XP partition to absorb the free hard disk space. You use the same tool if you want unallocated hard disk space back for future Linux installation.
You can also run Ubuntu as a Live CD without the need of being installed into a hard disk but it runs slower and saving files is harder.
Alternatively you can install Ubuntu onto a USB hard disk to run it from there, i.e. USB hard plugged in you boot to Ubuntu, no USB hard disk you boot to XP.