I just came back from the xmas holidays, and discovered that AB's forum still has the tree structure I had two weeks ago. The weird thing is that when I visited AB from my old man's pc, the forum was fine. I'm totally lost here and would appreciate any help.

Using:
XP
Firefox (NOT v2)
Adblock (various AB related items blocked, but I can't remember changing those before I first saw the tree structure)
NoScript (AB allowed, not globally)

Please ask if there's anything I haven't told that might help. Thanks a ton in advance.

That's easy, click on options button (below add reply/new topic buttons), and on the display modes tab click switch to standard, that should solve it.

or delete cookies=P

Thanks both. I deleted the cookies and now it's fine