Wireless LAN works without WEP, but not work with WEP

Hi guys, I have an interesting problem I have never encountered.
I can access the Internet perfectly via WiFi without WEP or other encryption.
However, once I turn on the WEP, which I am very sure is correct, the connection would be gone.
In fact, the connection between my laptop and my AP is still ok, it means that I could ping my AP from my laptop correctly,
but I almost can't ping any other site further than my AP because I get only a few respond packets, around one is 10 seconds .

I am very sure that DHCP and routing table are both fine. And any other laptop could access the Internet either with WEP or without WEP certainly. BTW, my laptop is IBM X60 with Windows Vista.
Have any one encountered similar problem? welcome any suggestion.
Thanks in advance.

Alex