Problem in virtual machine

I have successfully installed Virtualbox and have installed Win XP which is running well, except when I try to go on the internet. It suddenly stops opening most of the sites I want to visit, inclusive msn.com or microsoft.com, but have no problem opening google.com and can even perform the search and see the results, again without opening those pages either completely or in some cases it only show some part of the page.
I haven't made any changes in XP settings that can cause this. Does anyone know what the problem could be and how to fix it?

thanks in advance