i would definitely think about renaming your hosts file to hosts.old. then copy it to a new file named hosts.txt. edit hosts.txt so that the only line after the comment lines (the ones beginning with #) reads localhost 127.0.0.1
then save the file and rename it hosts with no extension (such as txt etc.)
alternatively you could download a host file from this guy
http://everythingisnt.com/hosts.html
but since his intention is to block ads you may not want to do that...
good luck
c_b