http-pages from other subnet unable to load

Hi There,

I have a strange problem on a Win XP SP2-Machine.
I Installed the OS, sysprepped it and sucked an image. Afterwards I
pushed the image to an identical machine.

Sysprep ran without any errors on both computers.
Now on a strange thing appears on the 2nd machine .
Everytime I try to access websites -which are not in the same subnet-
Firefox and IE are unable to load anything. I set up a foreign
http-proxy -which also isnt in the same subnet- and now that computer is
able to load foreign websites.
Unfortunately I cannot use that computer all the time with that proxy
and I need to fix this problem asap

Does anyone have an idea?

If you need some more information, tell me please