Windows XP: Network problem "No Default Gateway" and DNS problems

Network problem "No Default Gateway" and DNS problems

Hi.
I've recently moved home and after contracting an internet service I'm unable to connect to the network.
We have two laptops here:
- One is Windows Vista which is able to connect without a glitch;
- The other (my laptop) is a Window XP with SP3 which is unable to connect.
- The wireless modem is a THOMSON TCW710.

This is what I've checked:
- We have a wireless connection, but I've also tried connecting my laptop via cable and it doesn't work.
- My laptop is set for obtaining the IP address automatically.
- When I start windows, I get an IP address, the DHCP server and the DNS servers.
However I DO NOT get a Default Gateway.
- This is the output of the ipconfig /all command (I've omitted the bluetooth and VM ware connection to keep this short):

Windows IP Configuration
Host Name . . . . . . . . . . . . : jvargem-pt
Primary Dns Suffix . . . . . . . : pt.oracle.com
Node Type . . . . . . . . . . . . : Hybrid
IP Routing Enabled. . . . . . . . : No
WINS Proxy Enabled. . . . . . . . : No
DNS Suffix Search List. . . . . . : pt.oracle.com
ara
oracle.com

Ethernet adapter Local Area Connection:
Media State . . . . . . . . . . . : Media disconnected
Description . . . . . . . . . . . : Broadcom NetXtreme 57xx Gigabit Controller
Physical Address. . . . . . . . . : 00-15-C5-58-C3-A1

Ethernet adapter Wireless Network Connection:
Connection-specific DNS Suffix . : ara
Description . . . . . . . . . . . : Intel(R) PRO/Wireless 3945ABG Network Connection
Physical Address. . . . . . . . . : 00-18-DE-A5-16-7A
Dhcp Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
IP Address. . . . . . . . . . . . : 192.168.0.10
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . :
DHCP Server . . . . . . . . . . . : 192.168.0.1
DNS Servers . . . . . . . . . . . : 62.42.230.24
62.42.63.52
Lease Obtained. . . . . . . . . . : sábado, 5 de Setembro de 2009 11:11:14
Lease Expires . . . . . . . . . . : sábado, 12 de Setembro de 2009 11:11:14

- The Windows Vista laptop gets the exact same information as my laptop (with a different IP address naturally) but it also gets a Default Gateway.
- The Default Gateway IP is the same as the DHCP server.
- If I manually set my laptop's network configuration, also entering the default gateway, I still cannot connect.
- I have a Symantec Protection Agent 5.1 firewall. I've tried disabling it, but that didn't change anything
- I can ping the DHCP.
- However I can't open the modem configuration page (which is in the same IP as the DHCP server).
It works fine from the Windows Vista laptop.
- If I try to perform an ipconfig /renew, I get the message "unable to contact your DHCP server. Request has timed out.".
- When I use the Windows diagnostic tool, it complains about the DNS servers (can't ping them).
The fact is that the Windows Vista can't ping the DNS servers either, but its able to connect to the network.
- the Windows Messenger diagnostic tool also complains about the DNS Servers and gateway.
If I set the configuration manually, it only complains about the DNS servers.
- If I try on other networks, the laptop connects without any problems.

Can't think of anything else to tell you.
I've been looking into this and searching the web for the last two days and couldn't find anything.
I'm guessing there must be some incompatibility between the THOMSON modem and my laptop configuration.
Any ideas?
Thanks a lot.