DS2 did it - he was sent a link when he was chatting to a schoolfriend (not her fault) which he clicked on in all innocence; it was one of those which says something like "look at this!" and when you click on it something scary happens but not nearly as scary as what happens after.
It appears to have taken out both Norton anti-virus (clever... ) and AOL . I am currently using an old laptop on a dial-up connection and DH is taking the PC into work tomorrow to see if someone techy can fix it.
V v v pissed off.