Can't access certain websites
Look at your hosts file. You can find it by clicking on the "Go' menu in the Finder and select "Go to folder..." then type "/etc/hosts". Copy the file on to your Desktop and open it with Text Edit.

You'll see the following lines at the top of the file :

##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting.  Do not change this entry.
##
127.0.0.1   localhost
255.255.255.255 broadcasthost
::1             localhost 
fe80::1%lo0 localhost


If you have others lines underneath, look for the domain "svenskadomaner.se" in a line such as 127.0.0.1 svenskadomaner.se. If you find a line like this, then your Mac redirects the domain to your localhost (your Mac) and of course it won't find the server. This is the technique to block a website on a computer. So delete the line, save the hosts file and replace the "/etc/hosts" file with the one that is on your desktop.

Then you can try again to go the website ;-)

Edit : It's possible that you don't have "127.0.0.1 svenskadomaner.se" in the file but something like "127.0.0.1 IP Address" where IP Address is the address of the servers of www.svenskadomaner.se.