After running the following code, my computer freezes and alt + ctrl + F2 or alt + ctrl + pet sis/print + resuib don't work. I have to poweroff by pressing the power button.
Those are the commands:
root@bt:~# ifconfig wlan0 down
root@bt:~# airmon-ng start wlan0

Found 3 processes that could cause trouble.
If airodump-ng, aireplay-ng or airtun-ng stops working after
a short period of time, you may want to kill (some of) them!

PID     Name
1633    dhclient3
1768    dhclient3
1906    dhclient
Process with PID 1768 (dhclient3) is running on interface wlan0

Interface       Chipset         Driver

wlan0           Unknown         brcmsmac - [phy0]
                                (monitor mode enabled on mon0)

root@bt:~# kill 1633
root@bt:~# kill 1768
root@bt:~# kill 1906
root@bt:~# ifconfig mon0 down
root@bt:~# macchanger -m 00:11:22:33:44:55 mon0
Current MAC: --:--:--:--:--:-- (unknown)
Faked MAC:   00:11:22:33:44:55 (Cimsys Inc)
root@bt:~# ifconfig mon0 up
After that the computer freezes as I explained before.
I'm running Backtrack in an Acer Aspire 5736Z and the wireless card is Broadcom BCM43225