Ah, I took a look at the FAQ:
Q: What happens when the advert is served? Do you lose what was on your screen? For instance, what would happen if you were playing a game or in the middle of an online shopping transaction?
A: 60 seconds prior to advertisements being shown, a warning screen will appear, the screen contains an accept and delay button. Each advert broadcast can be delayed for 5 minutes to enable the customers to either pause a game or complete on-line transactions.
WTF!!!!
Q: What happens if I'm on holiday for 2 weeks? Will I have to watch all the ads in one go?
A: No, holiday periods are taken into account.
Ah, well, I take 12 x 3 week holidays a year...
Hmm, I was thinking of submitting a question to the FAQ - can I install my own OS, I would like to run FreeBSD and AROS, and maybe OpenBeOS, is this compatible with the adverts?
Hmm, thinking about it, could you get round the pesky adverts by booting an alternative OS from CD....?
The FAQ says that you have to watch 3 mins of adverts per hour of use, and that you are expected to use the PC for 30 hours a month, so I assume that means you have to use WinXP for thirty hours per month....
edit: So I wonder if they use the system clock to determine how long you've been using it? If so, you could have an advert watching session every morning - put the clock forward twenty minutes, watch an advert, put it forward twenty minutes watch and advert, put it forward twenty mins, watch an advert, and then boot linux... :-D