More Networking Troubles

Hey all,
I have an NB6 (Aussie Brand) modem and a Dlink DIR-300 router. I have a desktop(XP) that is connected to the wireless through a cable and my partner uses a laptop (vista) which connects wireless.

The problem I have been having lately, apart from really slow internet for adsl2 (around 1-2mb..) is that whenever the laptop connects to the wireless network the internet drops out? I've had network drop out issues before (I had another post on here a few weeks back) and now the most of it seems to be when the laptop connects to the network.
Sometimes we can just reset both router n modem n it work's fine whereas other times it just keeps dropping out every couple of minutes...

Anybody have an idea as to why this is happenin and how to fix it?

Thanks in advance!
Scy