Windows has stolen your master boot record (MBR) from you and over written grub (the boot selection screen).
What you have to do is restore grub to the MBR.
where x = the partition where Kubuntu installed /boot/grub.
note: partitions and disks are numbered from 0. So sda1 = hd0 in grub.
Chances are that you will have to then edit the selection screen list once you are booted back into Kubuntu. In /boot/grub you will find menu.lst you will have to amend Windows back in there. Because you'll need root privileges I'd recommend opening a terminal and using pico/vi/emacs. To open a terminal press Alt+F2 and type 'konsole'
So to edit i will
sudo pico /boot/grub/menu.lst
Ctrl+O to save and exit.
There will be an example windows boot option in there, so you can copy that and tack it to the bottom of the menu.lst file and edit it to match you computer. Save and try. Should look like this (modifyed for your windows install):
# This entry automatically added by the Debian installer for a non-linux OS
# on /dev/sda1
title Microsoft Windows XP Professional
P.S. Some good reading/tutorials can be found:
GNU Grub Manual
saikee's 'Just booting tips'
and of course LQ's own wiki