uninstall

a) - Be sure you have the Windows installation CD and it is readable from your cdrom
b) - Using Windows force a complete hard-disk check and reboot selecting Windows to perform the check
c) - Make an external backup (on cd or other devices ) of your relevant Windows data.

d) - Boot your pc from the Ubuntu live install cd
When Ubuntu is up press ALT+F2 and type into opened dialog window: gparted
Now using gparted
Select from upper right the /dev/... hard disk
Then select the etx3 right click with mouse on that partition and select delete, press "Apply"
gparted delete partition and close itself

Press ALT+F2 and type into opened dialog window: gparted
Redo the same for the swap partition.
gparted delete partition and close itself

Press ALT+F2 and type into opened dialog window: gparted
When you will see only the NTFS partition and free space
Select NTFS partition and resize it by dragging the cursor arrow then press "Apply"

Shutdown Ubuntu remove Ubuntu CD and insert the Window XP cd

Boot from Win XP Cd ( you will get setup inspecting hardware configuration)
Will come to screen to setup or repair existing install press R to enter recovery console
You will be asked to choose install to repair enter number and you will get prompt
for admin password followed by: C:WINDOWS>

at this point type FIXMBR or HELP will give you list of options.

Reboot the pc...

Depending on what drive you have XP installed you might have to give an additional command: "fixboot x:" where "x:" represents the drive XP is installed.

See: www.microsoft.com/documentation/bootcons_fixmbr
And: www.microsoft.com/documentation/bootcons_fixboot

HTH