Wireless Network Woes - Low Utilization

I have recently bought an FSG WLAN Gateway router/Nas to get it working with
a laptop and Xbox360.

I have got it working with the Xbox360 over a wired (via devolo homeplug)
network; the PC sees the files on the NAS; the Wii connects wirelessly BUT my
PC won't work very well wirelessly and I cannot figure out the problem.

I have the router working with WEP encyrption (rather than WPA so it will
work with the Wii).

The PC connects to the router and the internet. It shows that the signal
strength is excellent. However although I can connect to the internet the
connection is so slow as to be almost non-existent. Web pages take FOREVER to
load (for example Microsoft or the BBC site ). To fix this I have tried:

1. turning off the windows firewall;

2. turning off encyption;

3. changing the channel on the router;

4. updating the firmware on the router; and

5. updating the driver for the wireless network card.

As the Wii connected without much problem I think it is the PC.

It's dell Vostro 1700 running Vista Home Premium 32 bit with an Intel®
Wireless WiFi Link 4965AGN wireless card.

When I open the taskmanager and look at Network performance it starts okay
for a nano-second when IE7 opens and then drops to 0% and stays there.

Why does it do this and how do I fix it??