(By the way, I couldn't google the exact phrase, but found a few links about how to install Discworld II on WinXP. Well, well, well...)
Anyway, Skazz was perfectly right.
So I bought a fitball. It's like,
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No, I haven't punctured it yet :)
And no, it's not that hard to keep balance (it's quite easy to sit, at least :)
Yes, it's big and plum-coloured,
and yes, it's great to sprawl out on it :)