The HOSTS file is read with (and before) each DNS name request, so you don't have to do anything once you change it.
Lines that start with "#" are treated as comments to end-of-line, so just comment out the entries you want to "get through."
In WinNT, XP, 2000 -- the hosts file is found as /WINDOWS/SYSTEM32/DRIVERS/ETC/hosts
and in Win9x/Me it is found as /WINDOWS/hosts
Edit it with WordPad or NotePad (do not let either wrap lines or apply formatting) -- it is tough to open normally since it has no extension, from explorer you have to specify the app each time, or open NotePad and open it from there, or, better yet, create a desktop shortcut that tells NotePad to open the hosts file -- use a target like this: [C:\WINDOWS\SYSTEM32\NOTEPAD.EXE C:\WINDOWS\SYSTEM32\DRIVERS\ETC\HOSTS]
Then you can click on it to edit it whenever you like.