It took more than a couple of flu epidemics to make December 2000 the viral month it was. An advent which started on a voyeuristic bent, with a private email containing details of a couple's fellatial habits being "e-volleyed" around the world, ended with a deluge of corporate viral advertising on a scale hitherto unknown.

Never before have our inboxes been so assaulted with games, prize draws, gifts and a thousand other ways to kill time. They ranged from the genuinely engaging, such as Tongue of Flurry from Icon Medialab - where you have to catch snowflakes on your tongue and spit them out as ice cubes to waste evil snowmen - and freeloader.com's popular Soccermon, to the totally banal: MP3.com's all-singing, all-dancing greetings ("Let it Snow has been composed just for you"). But the bombardment left you thinking, as the season often does, why?

One thing is clear: conventional online advertising such as banners and mailshots to free or paid-for email lists no longer works, so companies are having to resort to smarter ways of promoting their brands. New-style approaches often exploit the British appetite for screen games - the UK leisure software market is now the third biggest after Japan and America. According to MXXI Europe, we now spend three times longer on entertainment sites than on sports sites and more than twice as long on them as we do on travel or retail sites. Last year the numbers visiting entertainment sites more than doubled between January and April and the average visit was 28.8 minutes.

Thomas Cook's snowboard game Powda, which runs until the spring, has been one of the most successful of the season's campaigns. Created for Thomas Cook by NetCandy, it is presented as a Christmas pressie by way of a weblink from TC to the 300,000 customers on its database, it challenges you to get down a mountain, avoiding any rocks, logs, trees and skiers. The winner of a prize draw gets a free holiday, and so far 25% of customers have played.

Alexa Clark, director of customer acquisitions, explains that the direct marketing exercise has threefold benefits. It increases brand awareness among the company's new target group, snowboarding youth, it helps to build up the database as it is passed around among friends (competitors entering the fourth level must pass on their details) and it sweetens up existing punters who think they are being given something for nothing. The high street travel firm says it generated 250,000 new customers through its last viral campaign in the summer, which offered a free booklet about weekend breaks in the UK.

Tangozebra, a technology-based sales and marketing solutions company, claims its Xmas Game, which was sent out to 600 industry and media contacts, had 65,000 hits within three hours. By clicking on a noisy parcel, you activated a banner game that required no plug-ins or streaming; you simply had to navigate Santa in his bobsleigh over the flying rooftops and avoid snowballs to enter a draw and win a hamper.

Martin Pavey of Tangozebra explains that retro games such as Pacman and the "flying defender" model are back in. This year's game campaigns for WCW wrestling and the Royal Bank of Scotland are based on that old viral favourite, Punching the Monkey. The former asks you to punch a wrestler, the latter a pound sign: if you hit the sign within a second you are told your reaction time is "excellent" and are urged to check out the bank's reaction time by hitting on another click-through. It's not exactly The Weakest Link, but it does at least engage impatient web surfers for more than a split second.

The days when people clicked on a flashing banner because it was novel are over, says Pavey. "The thing about email is that it's personal. If you can get some sort of effective reaction from people with a game, then you can start to get them involved."

He is working on a viral pre-campaign campaign for a major trainer company which hopes to use a cool game to tickle the details out of potential customers.

Now banner fatigue is setting in, conventional advertisers are having to learn youth marketing tactics, adapted to the consumers with the shortest attention-spans. Some amusing examples can be found at greenpeasoup.com (which enables those under-age to spend money on the web without a credit card), such as the e-postcards devised by WARL offering you the opportunity to microwave the heads of Smash Hits pop idols, and Splatt, the strangely addictive game in which you have to splat a fly before it makes a mess all over your soup.

"Viral campaigns have to be humorous and funny, otherwise people won't bother passing them on," says Adam Hartley, account manager at CruiseControl, which specialises in the kind of guerrilla and "street" marketing favoured by skateboarding and gaming companies. "Corporate logos are an obvious turn-off for our clients. If they smell a hint of a corporate sell, they'll just press delete."

To promote the Dino Crisis 2 PlayStation game, CruiseControl produced a series of spoof newsreels documenting the aftermath of a Tyrannosaurus attack on the M25 with crushed cars and 40ft-wide footprints left in the fields. Hartley traces its origins back to the viral impact of Bad Day (1999), a 10-minute film of tech-rage in which a man in an open-plan office starts trashing his computer. Staffers tend to make "subverts" because they enjoy them (remember the "I've got liver damage and my wife's just left me ..." send-up of Budweiser's ads?), but Hartley finds that corporates are increasingly interested in these formats as consumers become more jaded.

Now that "games" has overtaken "free porn" and "Yahoo" in the top 10 most searched words on the internet, according to Searchterms.com, it's no surprise that the industry is getting joystick-happy. And it's not just the kids who are boosting figures. Many of us office workers now prefer to spend our time popping digital zits or slapping a virtual Spice Girl than getting on with boring work, which is why so many businesses have embargoed open internet access.

The viral phenomenon implies that marketing values are changing again. The virtues of Restlessness, Impatience and Rebellion are starting to replace the time-tested aspirational approach built on the values of Loyalty, Glamour and Authenticity. (You only have to look at Thomas Cook's maudlin Tube campaign of rain puddles: "It's time to get out of the country".) Marketing gurus would give their hind teeth for the kind of viral notoriety that poor old Claire Swire unwittingly earned herself when her misguided email went global. Maybe next year the H&M underwear campaign will take the form of a "secret" videocam of Claudia Schiffer having phone sex with her latest celebrity boyfriend. Half the population of Europe can only live in hope.