WiFi vs. LAN question

I'm not sure where to post this:

I have a laptop with Windows XP. I also have a desktop with the
same. I installed a wireless router and got WiFi to work on the
laptop. My ISP is Comcast.

Some times, for reasons I don't get, the WiFi connection either isn't
available or just is so slow that I can't use it. At those times I
want to connect the laptop directly to the broadband. I shut down the
wireless router. I turned off the laptop, unplugged the broadband
wire from the router and plugged it into the ethernet port on my
laptop and started the laptop. When it came up I could not get
anything in the browser, even though the icon on the bottom right of
the task bar said the LAN connection was at 100mips.

Do I have to change something to make the laptop use the LAN rather
than the WiFi? I need to use the laptop for work. The software that
I use to connect to the office won't run on my desktop (a long story
for another time).

Help please!

Thanks