Start msdos (in run type cmd)Type : iwconfig

You will see your card as ath0 in manged mode. In order to stop that VAP u must use a utility call wlanconfig.

Type : wlanconfig ath0 destroy

Now if run iwconfig you should not see your ath0 VAP anymore. Now u have to create a new VAP in monitor mode.Here's how:

Type : wlanconfig ath0 create wlandev wifi0 wlanmode monitor

Now when you run iwconfig you should see your ath0 VAP in monitor mode.

Now to make sure ath0 is 'UP'

Type : ifconfig ath0 up

Now if you run ifconfig you should see your ath0 VAP in the 'UP' postion.Enjoy in aircrack