Problems connecting to internet

I have three computers using same internet connection. They connect through a D-Link Wireless G Router (DIR300) which is connected to a modem (nettcomm NB5 plus 4 adsl2+). I have been using the modem for almost two years and only 3 months ago started using the wireless router. One computer has had intermittent problems with this (of course, it is my favourite!). It has problems obtaining an ip address. Usually I reset or unplug the modem the router and the computer and this sorts itself out..... but not this time! Currently I cannot connect at all!! It still gives the same message that it has no ip address but i tried directly plugging in to both router and modem using ethernet cable and nothing. It acts like nothing happens. I have tried bothe firefox and explorer. The computer in question uses windows media centre as OS (which really is same as xp for most purposes). Any ideas??

Are two other computers OK?
You said, you tried to connect computer in question by a wire, straight to the modem?

Other two computers are perfect.. I am using one now on the configuration that I am trying to repair on troubled computer.  I have resolved the issue of not being able to connect via cable... so I can connect to either the modem or router directly and access internet but wireless still says "Limited or no connectivity"

Uninstall/reinstall network card/driver...

I am technically retarded... Can you please tell me how to do that.

Is it XP, or Vista?
Hold Windows key, and press Pause/Break key.

Thanks...
Microsoft Windows XP
Media Centre Edition
Version 2002
Service Pack 2

Go Start>Run, type in:
devmgmt.msc
Click OK.
Device Manager will open.
What do you have listed under Network Adapters?

1394 Net Adapter
Broadcom 440x 10/100 Integrated Controller
Intel(R) PRO/Wireless 3945ABG Network Connection

Download your driver from HERE
Don't install, yet.
In Device Manager, right click on Intel(R) PRO/Wireless 3945ABG Network Connection, click Uninstall. Restart computer. Windows will find it again.
See, if your connection is back. If not, we'll go from there, but tomorrow. Bed time

Thank You and Good Night (It's only 3:50pm here - I am in Australia) I will try and see how it goes.

It did not work... I will look for you tomorrow.

Go back to Device Manager, right click on Intel(R) PRO/Wireless 3945ABG Network Connection, click Properties>Driver tab, click Uninstall. Confirm.
Install freshly downloaded driver.
Restart computer.

IT WORKED! When I did it yesterday. It seemed to auto-install the driver and this time I double-clicked and went through installation wizard-so probably didn't even install yesterday. Only thing is as the troubled computer was my primary computer (and I only call it primary as it is my favourite and the one I set up network on originally) I will have to set up wireless network again. It now has gone back to default name of dlink with no security. So, one more question, do I need to uninstall old network details off others computers or, if I use the same config, will it be okay? Also, can you foresee any other issues? Sorry to ask so many questions at once. I have to go to work shortly and I figure that when I come home you will be asleep and this is when I will be sorting myself out, so I am trying to pick your brain in meantime.....

THANK YOU BRONI - I have been struggling with this for days, I am very grateful for your help and so glad I found Computer Hope.

Great news
I tell you what...create fresh Restore point, to start with.
Then start a new topic about your network configuration. I'm not that great with networking. Some other people here, are.
Make sure to post a link to this thread, so any helper knows what the situation is.
Good luck!