http://www.f-prot.com/products/home_use/win/ (probably cheapest, but still effective)
Any more recommendations, geekz?
This has been a public service announcement by the Society for Clean Networks.
(Because the next virus is only going to be nastier)
(Oh and get AdAware (http://www.lavasoft.nu/software/adaware/) too, for the more 'commercial' virii out there (thanks to nisaba for reminding me))