Vista - help on reinstalling vista home premium upgrade

help.... dell just replaced a defective laptop but ship it with xp since that
was the original os. i had upgraded to vista about 8 months ago. now after
getting my new laptop i am trying to upgrade it back to vista but the upgrade
option has been disabled. i can only do a clean install which i dont want to
since it would mean that i would need to load a lot of stuff back, some of
which i dont have. any ideas on getting the upgrade option to work??!!!!