Why is my network broken, although my computer has an IP address, and firewall disabled?

Why is my network broken, although my computer has an IP address, and
firewall disabled?

Question:
If a computer has been provided an IP address by a broadband router, and
the firewall is turned off, but internet nevertheless doesn't work and
it's not even possible to ping the router, then what might the problem be?
And how can I fix it?
Background:
I'm unable to connect from my desktop to [the wireless broadband router in
the appartment to which I've just moved], via [a wireless network USB
card].
The router gives me an IP address (first it asks me to specify a WSP PIN
code, which I do), but although I get an IP address, I'm unable to reach
the Internet: I cannot resove any host names, and I cannot even ping the
broadband router, i.e. the machine that gave me the IP address (!).
More information:
Below follows lots of hopefully helpful information.
My laptop, which has always been configured via DHCP, is able to connect
to the broadband router without any issues. Only my desktop doesn't work.
It recently had bridged networking configured.
Here are my desktop's interfaces, after I've connected it to the broadband
router. wlan0 (at the end of the list) is the network USB card.
$ ip ad
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast
state DOWN qlen 1000
link/ether 20:cf:30:4c:2f:02 brd ff:ff:ff:ff:ff:ff
inet6 fe80::22cf:30ff:fe4c:2f02/64 scope link
valid_lft forever preferred_lft forever
(Uninteresting stuff related to KVM, VMWare, VirtualBox:
4: virbr0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue
state DOWN
link/ether 6e:84:3c:57:95:4a brd ff:ff:ff:ff:ff:ff
inet 192.168.122.1/24 brd 192.168.122.255 scope global virbr0
5: vmnet1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
state UNKNOWN qlen 1000
link/ether 00:50:56:c0:00:01 brd ff:ff:ff:ff:ff:ff
inet 172.16.228.1/24 brd 172.16.228.255 scope global vmnet1
inet6 fe80::250:56ff:fec0:1/64 scope link
valid_lft forever preferred_lft forever
6: vmnet8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
state UNKNOWN qlen 1000
link/ether 00:50:56:c0:00:08 brd ff:ff:ff:ff:ff:ff
inet 192.168.133.1/24 brd 192.168.133.255 scope global vmnet8
inet6 fe80::250:56ff:fec0:8/64 scope link
valid_lft forever preferred_lft forever
END uninteresting stuff)
9: wlan0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP
qlen 1000
link/ether 44:94:fc:29:59:6e brd ff:ff:ff:ff:ff:ff
inet 192.168.100.6/24 brd 192.168.100.255 scope global wlan0 <-- it
has an IP addr (!)
inet6 fe80::4694:fcff:fe29:596e/64 scope link
valid_lft forever preferred_lft forever
Here is the routing table: (192.168.100.* is the broadband router's network)
$ route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
default 192.168.100.1 0.0.0.0 UG 0 0 0 wlan0
link-local * 255.255.0.0 U 1000 0 0 wlan0
172.16.228.0 * 255.255.255.0 U 0 0 0
vmnet1
192.168.100.0 * 255.255.255.0 U 2 0 0 wlan0
192.168.122.0 * 255.255.255.0 U 0 0 0
virbr0
192.168.133.0 * 255.255.255.0 U 0 0 0
vmnet8
Here is the iptables configuration: (it's disabled, right?)
$ sudo iptables -nL
Chain INPUT (policy ACCEPT)
target prot opt source destination
Chain FORWARD (policy ACCEPT)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
When I ping the gateway (192.168.100.1), and Google, ping says:
$ ping 192.168.100.1
PING 192.168.100.1 (192.168.100.1) 56(84) bytes of data.
From 192.168.100.6 icmp_seq=3 Destination Host Unreachable
From 192.168.100.6 icmp_seq=5 Destination Host Unreachable
From 192.168.100.6 icmp_seq=6 Destination Host Unreachable
$ ping google.com
ping: unknown host google.com
How is that possible? 192.168.0.1 has given me an IP address, then it
cannot be "unreachable"? I'm able to ping the gateway from my laptop, so
it does reply to ping requests.
Here is resolv.conf: (Why does it specify localhost???? — I suppose that's
(parts of) the reason Google isn't ping:able?)
$ cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by
resolvconf(8)
# DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 127.0.0.1
In the past, I had a bridged network configured in
/etc/network/interfaces, but I've commented it out, and restarted the
whole computer. Here are the not-commented-out lines in that file:
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet dhcp
I'm using Kubuntu 12.04 LTS. Network Manager shows wlan0 as status CONNECTED.