internet on laptop works for like 2 minutes then stops

I used to always be able to access our wireless, no problem. but when we recently added a WEP key to our internet, things stopped going so good. we have 3 computers in our house--2 laptops and a desktop. one of the laptops and the desktop connects to the internet without issues. of course, myyy laptop is the one with problems. I can connect up for less than 5 minutes, and then it stops working. sometimes when I try to reconnect it works, but other times it doesn't. when it does work, it says at the top that it's not connected BUT on the description part it says "you are currently connected to this wireless blah blah blah".
I don't think it's the signal strength or anything... the desktop is the farthest computer away from the wireless box thingy and that works fine. please help! i'm so sick of using this desktop. I know I have the right WEP key, since it connects.
my laptop is a Dell Windows XP