You don't tell in what environment.
If at work, it's probably due to an access policy, or proxy rule, that blocks those, and other, sites as not-work-related.
In that case, only your network administrator can enable the access, as he have disabled it. You should also get your boss concentment before contacting the network administrator.
If at home, check your host file. It's located in \windows\system32\drivers\etc\ and can be edited as a text file.
Look at lines like
127.0.0.1 www.facebook.com and
127.0.0.1 www.msn.com
Add a "#" in front of them.
Save the file.
Electro, April 2011
