Geoff,
Are you actually asking about Windows®Defender, as it's a free product
available to licensed Windows users through the Windows Genuine Advantage
validation program? (so paying, sad to say, isn't an option, though we
love the £)
http://www.microsoft.com/athome/security/spyware/software/about/faq.mspx
"Windows Defender (Beta 2), subsequent beta versions, and the final release
version will each be available at no additional charge for currently
licensed Windows customers. You will be required to verify that you are
using genuine Windows through a quick and easy online process called
validation."
Instead, perhaps your asking about subscription based Windows Live OneCare.
That shipping released product is available for trial but does expire after
a 90 day period (version 1.1 - subscribable by US based customers only), or
as a 180 day trial in the International Beta version (v1.5) which was
recently made available. 1.5 includes a subset of Defender within it's
interface rather than as a separate product, it supports Vista, and is
available from here:
http://ideas.live.com/