Can't get to certain websites?

Hi I got a wierd problem.

I can't get access to cetain websites, such as youtube or myspace or connect to
msn messenger. Either with firefox or safari.

I got a macbook pro,
Not using a router, wireless or a firewall.
I've tried a lot of things like reinstalling softwares and "cache out X"

Internet works fine from the other computers in my home(PC).

What bugs me is that I can access the websites if I use anonymouser(?)

Please help me!