In your original PM you mentioned it reverting to localhost after entering a user/pass combination. I assumed you had a CMS of some kind.
I can access only my home page from the Internet but as soon as I login or try to browse my site it changes from Internet to Localhost. (You know in Internet Explorer at the bottom of your browser you have a little image of the world ? well that changes from Internet to Localhost)
Can you give me your URL so I can take a look at what is happening. This is important as without it I won't be able to help you. I can only guess at what is wrong.
After installing did you click on xampp/setup_xampp.bat
Did you empty your IE browsers cache?
When doing test work on your web site or server you should set your browser to refresh pages on every visit then empty your cache, as the cache is not your best friend in these cases.
To do this in IE click on:
In the General Tab under the heading Temporary Internet Files
Under Check for newer versions of stored pages
Every visit to the page
Click on OK
Then click on Delete Files..
under Temporary Internet Files
Don't tick the box just click OK
Now visit your page again from the Internet.