DNS issues with USB modem on windows 7.....

Hi, i have a USB modem through which i connect to internet on my two laptops. One laptop has windows xp sp3 installed on it and the other one has windows 7.

Recently the windows 7 laptop started having some dns issue. The problem started a few days back, while using internet on my machine, google chrome started showing error 105 "Unable to resolve the server's dns address" for some pages. This wasn't happening all the time, but was happening regularly and the behaviour was not specific to any particular website. In IE 9 i was getting the error "page can not be displayed".

I tried all the fixes mentioned in the forums and also tried scanning for viruses using malwarebytes and avast and was finally able to resolve the issue by changing dns settings for my internet connection. I now use google dns addresses to connect to internet.
8.8.8.8 and 8.8.4.4

So far everything is working fine but i am still wondering why this happened in the first place and an even bigger question is, why the same connection is working fine on the other laptop running xp sp3 and doesn't need alternate dns addresses to display web pages?

Please help..........