Web browsing semi-disabled?

[sys]

Hiyas,

Am new, very new, to ubuntu... Installed ubuntu 8.04 last night and this
problem wouldn't go..

All seems well in terms of internet connection, IM chat is fine, DNS and
ping are fine -
apart from firefox only able to connect with a few sites..

I can get google.com for example - but not ubuntu.com. I am able to got
connected with: diamondcard.us (ekiga), and the bbc (front page and weather
- but not news..)

After a bit of a search on the ubuntu help site, it seemed that maybe my
problem was solvable by adding the line net.ipv4.tcp_default_window_scaling
= 0
to etc/sysctl.conf 

I added the line to the top of the file - but it didn't make any
difference.

Any idea what may cause this behaviour and how to fix it..?

Not sure if relevant:

The machine is dual boot.
Internet connection is via ethernet cable to a router.


Many thanks!


Ron