Since you have Ubuntu 8 installed, i guess you know fair deal of the installation process.
Just install the new Ubuntu 10 on the old Ubuntu partition and it would recognise XP and add then in Grub Boot Menu.
JUST CHOOSE THE PARITION CAREFULLY.
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Anyways these are the usual steps:
1: Boot with Ubuntu CD
2:
Choose "Manual/Custom" during Parititoner option.
3: See where your previous Ubuntu was installed and select that parition and
choose "Edit" and choose Mount Point "/" (root parititon).
4: Check if Swap parition is already selected (you would already have a swap parition since you have old Ubuntu installed already)
5: This would write the changes to the Disk and format the selected root parition and installation would commence.
6: It would install the Grub bootloader and it would add Windows XP or other Linux installation in the Grub Boot Menu.
7: Reboot
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I know reading such things makes it scary to install Linux but it is plain and simple.
Carefull while selecting the parition since its names are different in Linux than Windows. (/dev/sda etc)
*www.howtoforge.org/the-perfect-desktop-ubuntu-9.04
