Problem - Computer cannot access internet (well not exactly.....)!.

I have the following situation and it's driving me nuts!! Any
suggestions would be most welcome.

Configuration: Wireless home network; 1 laptop running WinXP, 2
desktops running Win98SE. All local communication good and one WinXP
and one Win98SE perform everything OK.....BUT

I have one Win98SE desktop that runs fine EXCEPT that it exhibits the
following behaviour:

1 - sharing: even though the other two computers can see this PC OK on
network neighbourhood, they cannot access any resources on it even
though everything is set properly. This computer can see the local
workgroup through network neighbourhood and access resources on the
other two computers.

2 - When copying files from or to it from one of the other two
computers (using explorer drag and drop) it is incredibly slow!

3 - internet access - even though the the other two computers on the
netowrk can access the internet just fine, this one can never reach a
web page. It does seem to get the typed url resolved to the IP address
OK, but never is able to display the web page. I have tried IE6,
Opera, and Firefox with same result.

I have tried with and without various different software firewalls. I
have swapped the two wireless network cards between the two Win98SE
computers and the problem persists, so I don't think the card is the
problem.

I can ping a url OK from this computer. I can ping this comnputer
from either of the other two on the home network.

Any ideas please?