You've been hit by malware.
If you cannot amend the HOSTS file then you have been hit by malware.
But changing HOSTS should be simple if you remember one important thing. HOSTS does not have a file extension. If you open HOSTS in Notepad, delete the entries then save the file, it is likely to be saved as a.txt file. The browsers do not look for HOSTS.txt.
To re-save as HOSTS you use Save As in the File menu, make sure the name does not have a file extension, then make sure the "Save As file type" is changed to "All files" and not .txt files.
If you open HOSTS up again in Notepad and nothing has changed, see if the file has been set as "Read only" in it's Properties. If so, change that and try again.
In any case, you need to run a full scan with your anti-virus and also with the free versions of Malwarebytes Anti-malware and SUPERAntiSpyware.