WEP over wireless network causes loss of shared internet connection

I have two machines, one of which (A) is connected
directly to a DSL line and the other (B) which has a
wireless NIC. I have a USB wireless NIC that is plugged
into A, and I am able to set up an ad-hoc network between
A and B that allows me the share the DSL connection with
B. HOWEVER, if I try to use WEP on the ad-hoc network
when I do this, the internet is inaccessible from B.
I've tried fiddling with just about everything and still
can't establish a secure wireless network for connecting
B to the internet. Any thoughts/ideas?

Thanks,
Josh