It's for those poor stupid idiots who wish to 'upgrade' a recently bought copy of XP to Vista.
It's possibly the most spectacularly inept piece of web coding I've seen in a long time. It takes about 5 minutes per page, and just selecting an option on a page seems to require a whole sequence of talking back to the server to get another bit of data. Hey, even I know better than to do that, and I've only been doing ASP for a week!
(It previously got as far as taking my address before falling over. This time, it had finally decided that my voucher code hadn't been used after all, re-accepted it, and got as far as taking my credit card details before then dying.)
EDIT: My goodness, it's finally got through! And this time it didn't 'burn' the voucher number. Maybe the downtime they had earlier was actually for a bugfix or two.