Data

Offhand, I would say your IP blocked.
Other reasons could be that your machine is blocking these sites as they are considered bad. They would normally be added to the internet Explorer blocked sites list.

Do you use a proxy allready? Maybe without realising it and that proxies IP features on a blocklist.

Your firewall may come with a list of defined bad sites.

It depends what measures are in place as regards blocking at your end and who and what you surf through.

More Info is needed I'm afraid.