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If they made it quite clear as to their requirements, ie "The televised pentium 4 £799.00 offer", and the sales assistant sold them something that doe's not compare to that, then I would think that they have a claim against the sales assistant and PC World. Selling a fist full of add-on's, bundled software and duplicated 'anti's' doesn't sound very customer friendly like. Before any further delays [time is the essence, for a quick resolution], I would seriously consider further actions that may need answers, especially on the substituted computer and its comparison to the model requested.
There is bound to be someone who will have the answer of the two computer specifications, and their advice will be paramount on the decision of a possible returns.Sorry that I cannot be more helpfull.