wireless - WI-FI doesn't work (Hard Blocked)

Yesterday I installed Ubuntu 12.04 for the first time (I've always been a Windows user) as I was at the office and I had the Ethernet connected.

At home I only use WI-FI. When I started Ubuntu it didn't connect or try to connect with any network.

By google, I found the command rfkill and these are the results:

0: hci0: Bluetooth
    Soft blocked: no
    Hard blocked: no
1: brcmwl-0: Wireless LAN
    Soft blocked: no
    Hard blocked: no
2: dell-wifi: Wireless LAN
    Soft blocked: yes
    Hard blocked: yes
3: dell-bluetooth: Bluetooth
    Soft blocked: no
    Hard blocked: no

Well, the WI-FI is blocked by hardware, so I tried the command rfkill unblock 2 and it didn't work. As I didn't know even my Broadcom model, I found lspci and the result for the WI-FI is:

0e:00.0 Network controller: Broadcom Corporation BCM4312 802.11b/g LP-PHY (rev 01)

My laptop is a Dell Vostro 1520 with WinXP also installed.
Thanks for the help!

PS: It also may help that when I enter System Settings > Network (it may be different, my Ubuntu is in Spanish) "Airplane Mode" is connected. When I try to unlock it, it goes to its original place after I close that window and open it again and when I reboot.

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Update added from comment to an answer

The Broadcom "Additional Drivers" are turned on and enabled.

Broadcom and ubuntu do not play nicely together

Try these fixes

Installing Broadcom Wireless Drivers