Alternatively, You can dual boot.
Back up Your files, Format the entire HD, create two partitions.
C:\ & D:\
Install XP on both partitions!
Boot into the running XP installation on C:\ and pop in a Windows Vista DVD and upgrade!
Assign at least 15GB to the partion Vista will be installed on. (I Recommend).
The Vista Boot Loader will add the existing XP installation to its OS list, it will be titled;
"Earlier Version of Windows"
You can use EasyBCD or Vista Boot Pro to rename "Earlier Version of Windows" to "Nairaland XP" or "XP Service Pack 15"

If You will like to retain the existing Vista installation! You can shrink the existing partition to create space for XP!
Refer to the link below:
Open a Google window and type:
inurl:www.raymond.cc Dual Boot Vista/XP
That blog has all You need to get You going!!