When I enter a sub domain address "http://group.company.com" the address is changed to "http://company.com" and the company site is displayed instead of being directed to the sub domain.

Any help will be appreciated.

I uninstalled IE8 and had the same sub domain access problem. I reinstalled IE8 and was able to access the sub domain 1 time. After I exited the sub domain I have the same access problem as before.

There is nothing for your browser to deal with. There is some problem either in your DNS or in your web server. Do check with that and confirm that you have not forwarded group.company.com to company.com OR that subdomain exists OR if exists, it is pointing to the right server.