I have just installed the IIS so I can view asp files in a browser but when I put the address in a browser as : http://localhost/index.asp I get an error.
The error
OK, working from memory here as I am not in front of a Windows machine.
If you right click on your webroot folder /inetpub/wwwroot/ or the website directory you are working on open properties and select security, I think it is, you will see the list of users with their permissions for that folder. There is a section to add new users where you can add the IIS_IUSRS account (search from the list of users if you need to) which will be the default user used when anonymous authentication is enabled. Give this account the relevant permissions (read, write, execute) ensuring you apply to file and subfolders. Refresh the website in IIS and you should hopefully be good to go.