IIS prompted me to enter username and password.

i have to enter username and password when run http://localhost/ at mozilla firefox. but my iis works fine at IE. why it happened? i never experienced this thing before. i never set any username and password at my desktop. i have removed apache from my desktop also. how to solve it?

I see this when trying to visit the root web on XP. One fix is to simply add the IUSR_computername user to the permissions for the root web folder and files.

i have done what u said but still can't solve it.

Sorry, I've never pursued this problem, all my webs are in virtual directories and FF works fine there. I'm sure it's some permission-related thing caused because FF doesn't support NT challenge-response authentication that's built into IE.

Doug's right, it must be a permision problem.
Have you granted read/write perms to the IUSR account?
I've seen this problem with images that the IUSR does not have permission to read/write. Usually I just make sure that the IUSR has read/write permissions to the web folder in question and make sure that the "Inherit from parent the permission entries that apply to child objects. Include these with entries explicitly defined here." checkbox is checked.