Open and WEP don't work, WPA just fine

I have a small wireless network at home, it's primarly used by 3 laptops.
Two of them support WPA and the other just WEP

When I change the access point to WPA, the two first computers connect just fine but the other one (of course) doesn't connect at all

I moved to WEP to keep compatibily, this way I get it working partially. The internet becomes really slow, the two WPA-enabled laptops connect to the network for 5 minutes and then the link is dead

I tried disabling encryption at all but the same as WEP happened, any clue?

I don't know if the problem is with Windows because Linux connects just fine but I need to "wake up" the card once in a while