Wireless problems - but only one machine/network combo

I have 2 XP Pro machines and 2 networks (one on Linksys router and one on
D-Link router) in different locations.

Both machines work perfectly when connected by wire to either machine.

One machine works perfectly when connected wirelessly to either network.

The other machine works perfectly when connected wirelessly to the Linksys
network, but very poorly when it attempts to connect wirelessly to the
D-Link router - OK sometimes on the internet, mostly only gets to a few web
sites, generally very slow and never connects to Messenger.

Would appreciate any de-bug advice.

Thanks,

Steven