Wireless connection connecting and disconnecting all the time

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http://unix.stackexchange.com – I am running Ubuntu 11.04 on my laptop and I use a 3G USB modem for my Internet connection. I wanted to share this Internet connection with my girlfriend's laptop that is running Window XP. Basically, I created the the wireless connection on my laptop and now it just connects and disconnects all the time. Why is it doing this? I am connected to the Internet via the 3G USB modem while I am also trying to connect the wireless set up, could that be the reason? Is there anything I should do on the Windows computer during the wireless set up? (HowTos)