They've implemented DNS hijacking, so if you mistype something in a web browser, you get to a page of adverts instead of an 'unknown host' error message. To opt out of this, visit:
https://my.virginmedia.com/advancederrorsearch/settings I've seen claims that there's porn in the results, but I can't comment because I opted out before examining the search results.
If you do anything other than web browsing then it can really screw that up because it'll return the address of their ad server rather than an error if you mistype a hostname, and non-http traffic won't talk to their web server.
Sensibly they opt you out on the basis of your cable modem MAC address, so it'll take your entire home network out if you've got more than one machine. Obviously it also works for everything and doesn't rely on a cookie.