Specific websites not loading, strange download behavior after upgrade

Ok, so this is a weird one that I've never ran into before and I wasn't sure how to search for anything related to this as the problem is very difficult to explain but here goes.

I have a local domain setup at the office running off of SBS 2008. Before a couple weeks ago, all the client machines were Windows XP. We purchased 3 new workstation from dell preloaded with Windows 7 Pro. Since we hadn't had any Vista/7 machines on the network connected to the domain no group policies had ever been made. I was able to setup the firewall and internet explorer settings just fine. But we are still having serious issues that I can't seem to find any information on.

1) I've found certain website that are blocked only on the windows 7 machines: dell.com and mozilla.org are some examples. I thought dell might have been related to mixed content but that didnt seem to fix it. Also, it doesn't actually block the site in any way that shows an kind of error, it just never loads. The title of the page will change and then just sit forever.

2) Other application that require internet don't work. I was able to download the firefox installer from another computer and transfer it to one of the windows 7 machines. But once it tries to download the program it never starts. Neither does it time out or say it doesn't have an internet connection.

3) I was able to take one of the machines home and the internet worked perfect. I installed firefox on the machine everything was good. But then when I went back to work the following day I had the same issues in FF as I did in IE. Not only that, downloads would start just fine and download about 150KB then suddenly stop. It wouldn't error or anything just stop, even the speed wouldn't update to show that it had stopped. Just the completed wouldn't change anymore. Even more weird, if I pause and then resume the download it would download some more, between 100KB-1Mb worth then stop again. Until I pause and resume it again. (Very painful way to download files)

So I believe it has something to do with the way the domain is setup. I was thinking it was a group policy something because I know windows 7 uses different policies than XP. However I can't seem to find or even think of anything that would cause this behavior.

Any help would be very much appreciated.

I am the "IT guy" for another company and they asked me for my input. No official training in anything, just much more experienced than the average user. I do not have much experience with group policies. Enough to know what they are for and how to change a specific setting im looking for but not enough to know where to look for something like this.

It sounds like the DNS isn't set up correctly.

It may help to set up the Doman Controller as your preferred DNS server in connection properties.

Ya I know its not anything simple like that. And my primary DNS is my domain controller. And if it were DNS it would be affecting the XP computers as well wouldn't it?.