Does it work if you try it using this address
http://127.0.0.1/phpmyadmin/
If it does then its probably that you dont have a correct HOSTS file.
c:\windows\system32\drivers\etc\hosts ( no extension )
should have the following line
127.0.0.1 localhost
--------------------------------------------------
The hosts file is protected by windows, in order to save it you must have Administrator privilages.
Vista/W7/W8 you may think you are an Administrator BUT YOU ARE NOT.
To successfully save the hosts file do this to launch your editor with Admin Privilages.
Locate your editors icon on the desktop or from the Start menus ( notepad will do if you have nothing else )
left click + shift over your chosen editor icon - will show a menu.
select "Run As Administrator" from the menu.
Navigate your editor to the c:\windows\system32\drivers\etc\hosts file.
When you have made changes you will now be allowed to save them.