Web Sharing in IIS

Hello.

I have a little problem with sharing files in IIS. Maybe somebody can help me.

I have WinXp with IIS installed, and I want that the wwwroot directory will be available for access from the internet (by the browser). When I'm connected to the internet and I type my computer's IP, I get a popupped window with a request for username and password. Why ? And where can I edit the usernames ?

P.S.
Is it safe to open the computer for access ? Can people steal files from the wwwroot folder ? And if someone that has a username for access can steal the files ?

Thanks.

yes, same question.
waitting people hand!