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