Just reformat that partition with Gparted
^^ I know but Gparted gives you more options
Open Start Menu -> Right Click Computer -> Click Manage -> Click Storage -> Click Disk Management -> Right Click the XP Partition and Select Delete.
To delete the XP option from the boot menu, Open Start -> Type Msconfig into the search box -> Click Msconfig -> Select the Boot tab -> Delete the XP entry
You can do it within Windows.
Star--Run--cmd (Enter)
Bcdedit /delete –f
it will delete all your old windows xp ntldr.
but i suggest you reinstall your windows vista after reformat