I assume that large parts of the network had been flooded with packets of information both from people trying to bring ZoneLabs down (sites still not available) but also from people trying to visit the ZoneLabs websites. If an ISP's DNS was trying to locate ZoneLabs websites it would've had problems doing so increasing the time it took to do anything else.
So Blueyonder (my ISP) would have had it servers looking for the ZoneLabs website and hence my requests for PCA site, Dilbert etc etc would be waiting at the back of the queue (as it were) whilst BY is trying to find ZLabs!!
However I'm curious about this comment - Not a very good advert for a security company!
Could ZL have done something to prevent the DDoS attack? Or is there something they could / should have done when it started?
The people who started this attack off have no doubt found a flaw in a system to be able to do it. It used to be true that you could do a 'ping of death' against a system and that would slow it up/bring it down. Systems can now be protected against such things.