Your question indicates you no longer need Ubuntu on this machine and that space will be reclaimed for the use of windows XP.
You can eliminate the partitons from windows XP itself. As a precaution, take a backup of windows data before doing the steps.
Go to 'Settings -> Control Panel -> Administrative Tools -> Computer Management -> Storage -> Disk Management' from windows xp and simply delete and reformat the ext2/3/4 partitions which will be Linux partitions and create and format new windows partitions.
I assume the primary partition is windows XP and it is the one with boot flag.
Now you will need to fix MBR to remove grub and re-instate pure windows XP MBR.
Refer http://helpdeskgeek.com/how-to/fix-mbr-xp-vista/ (I personally did not use this in the past, but after reading it and remembering my past experience this documentation looks correct).
Now you should have PC without old Ubuntu.