Wireless Networking SLOW

I am working on a "white Box" system with the following
specs.;

Asus P4PE Main Board, P4 2.8 Ghz CPU, 1 GB Ram, 80 GB
Hdd, ATI 9700 AGD Video card, Linksys WMP11 Wireless Nic,
RTL 8139 class Network card for cat5 connection.

The system is running WinXP Pro with IE6 SP1.

When ever the customer connects via the wireless
connection, the internet pages take extremely long to
load, I am talking like up to 5 minutesm but when they
connect via cat5 cable and a direct connection they are
normal for a Cable Internet service.

The customer has nother OLDER system right next to this
one running the identical Wireless Nic and it blows this
one away, like 30secs to load web pages when connecting
wirelessly.

We have tried uninstalling and reinstalling the card,
updating the drivers, rolling back the drivers, even
turning off the other system to prevent any interference,
point of reference this setup was fine until recently,
and still cannot resolve this issue!!

Any ideas would be greatly appreciated!!

Thanks in advance,

Dwight