Kaspersky Internet Security 2009 Renewal - 1 Year, 1 User £5.99 from eoutlet
Never used them before, but reviews online seem okay.
Yes, this software is free if you use Barclays online banking, but if you don't this is the cheapest I've found for a renewal license (trial software freely available to download from Kaspersky's website).
Free to upgrade to the latest 2011 version, so all you're really buying here is a license for a year.
Google Checkout too which is a bonus.
Yes, there is free software available that will never find any viruses and give you lots of false-positives, but that one's been argued to death thousands of times before.
Edit: Price now back down to £4.99 and updated title to show that it is upgradeable to 2011.