wlan connection becomes very slow if other system is connected

[RedAnarchy]

if someone uses the connection (via wlan, other system runs xp) my wlan is very very slow. it's normal that it becomes slower, i know, but it takes nearly 5 mins to load a page in firefox, yum doesn't work and sometimes i get disconnected. when i disconnect the other system, everything works fine again. is there any priority-like thing or what could cause the problem?