Hi stephenc,
Regarding Windows hosts file. You must follow this path: C:\Windows\System32\drivers\etc\
where "etc" is directory. Inside "etc" you can find the file called "hosts". Open it, using
Notepad or any text editor, and check if there are changes compared with default configuration (like in attach image). In many cases, the "hosts" file is object of malwares which modify the basic configuration.
You can try DNSJumper mentioned by abruptum, but IMO, you are using the same ISP, so if the DNS would be the root cause, both computers should have same symptoms. Anyway, rule out this option.