modify hosts file

One of my clients PC needs to change its hosts file, but can’t change it. After I modify and save it, I get error message ” cannot create the c:\windows\…\etc\hosts file, Make sure that the path and filename are correct.”
Then I found that it’s read-only, and I uncheck its “read-only” attrib, but always going back to check so not working. I’ve scanned any virus or spyware, but nothing found.
Could you please tell me how to modify the hosts file successfully?
Thanks for you help.