Windows will overwrite your master boot record on install without asking or notifying you so that is the problem. You need to either configure xp to boot Ubuntu or update Grub2 to boot both from Grub.
I haven't used Grub2 but if you do a search here at LQ, you should find a number of posts for updating Grub2. Since you can't boot Ubuntu, you can use your Ubuntu install CD.
In the link below, section 4, C explains it.