This is only 1 item to check for a possible fix for no localhost and/or 127.0.0.1 access on the PC that XAMPP is installed on.
file in your text editor and look for:127.0.0.1 localhost
If it is not included then add it as the first line after all the commented lines.
Also if you have this line:::1 localhost
Comment it out so it looks like this:# ::1 localhost
Now the first 2 lines should look like this:127.0.0.1 localhost
# ::1 localhost
Save the file and try again to access http://localhost
Also check that firewall access is allowing access and virus scanners are allowing access - temporarily disable these to test.
In Vista make sure you have disabled UAC and you are logged in as an administrator or a super administrator
And of course make sure that Apache and MySQL are both actually running.