Internet very slow since router install

[Foxylady]

Hello all,
i put in a router modem namely the Belkin N150 adsl into my friends house so he could have wireless for first time.
He is with Talk talk of all peeps, man i love virmin lol.
He has a Sagem modem which the new router is replacing for the net access, all done and working, Except the internet is really slow wether it be via the ethernet to his pc or wireles to his laptop from the belkin.
So to eliminate cable wiring, filters etc etc, i simply put back in the sagem original modem back and connected to the net on the pc and bam the speed back to normal, soon as connect with belkin its damn slow, less than half a meg.
Is their a setting i have missed out in the router config or is it the router is naff?
Thanks for any advice in advance
Foxy