You would have had to have an anytime upgrade disk to do your original
upgrade. The Windows Vista disk includes the anytime upgrade.
You will have to have an activated Vista Basic version on your computer then
redo the Home Premium upgrade by using your license (or install key) that
has been stored for you in your digital locker online at Microsoft.
If you format your HD, you then will have to restore Vista Basic by the
method the laptop manufacturer has provided. Then you can upgrade from there
after you have reactivated you Vista Basic copy.
You mention that you do not have a disk, but a disk was required for you to
do the upgrade to Home Premium that you said that you had done?
If you mean you purchased an upgrade, then you should have also purchased an
anytime upgrade disk at the same time you bought the upgrade license
Alternatively you could get a hold of someone else's copy of Vista to use
for the anytime upgrade.
If you have SP1 installed, you might have to uninstall it first, or use the
new Vista retail disks that have SP1 integrated.
