New Laptop Slows Existing Wireless Network?

Anyone have any ideas why getting a new laptop with wireless would slow
down my existing wireless network? Here is the situation. I have been
running a Belkin 802.11b router and a Dell Dimension desktop with
Belkin 802.11b PCI card for over a year with no problems. I
occasionally also use a Dell Latitude laptop with internal 802.11b and
it also has always worked fine. WEP 128 bit encryption is turned on.

Now the problem. I just got a new Dell Latitude laptop with internal
802.11b/g wireless. It works just fine. Everything else also works
fine as long as the new laptop is sitting right next to the older
computers. However, if I take the new laptop to another room, the
wireless signal on the desktop computer goes to zip, acording to the
Belkin monitor. The desktop's network connection barely works. The
new laptop continues to work fine. This problem does not happen if I
take the older laptop to a different room.

I tried changing a few settings on the new laptop, RTS/CTS mode and
802.11b only, but it did not help. Any ideas? One question: do you
have to reboot after changing the wireless settings? There was no
indication so I did not reboot.

Thanks.