Dell have never sold computers via any other company. They used to sell via their own High Street outlets but they've now closed their network of shops down and only sell online. So I'm completely mystified as to how on earth you managed to buy a Dell product through PC World!
Kgmrush has provided the relevant phone number but you might also find some help here:
http://support.euro.dell.com/support/
It might be worth posting another question, telling us exactly what's wrong with your PC. It might be something (such as a virus or a software problem) which Dell will, not unreasonably, deny all responsibility for. We might be able to help you find a solution to your problems.
Chris