BlueLife Hosts Editor is a free portable editor for your HOSTS file (a document used to map domain names to IP addresses).

Launch the program and, after a few seconds, it displays your current HOSTS contents. This can be useful in itself as the file may be used by malware to redirect web access from real sites to their own.

If you spot a problem then it's easy to delete some or all of the entries.

A Backup Manager enables you to save the current HOSTS file and restore it later. (The program comes with an "Original Hosts File" backup which restores its original configuration.)

There's even a toolbar where you can enter a domain name, resolve it and add it to the list in a click or two, preventing others from accessing it on your system.

(You may not see the effects of this immediately, but the program has tools to flush your DNS cache, or restart the DNS Client Service, which may help. If not, try rebooting.)