Java is a programming language and computing platform first released by Sun Microsystems in 1995. There are lots of applications and websites that will not work unless you have Java installed, and more are created every day.
IIS is able to impersonate the IUSR_<computername> account because the user name and password for this account are known by IIS. Before returning a page to the client, IIS checks NTFS file and directory permissions to see if the IUSR_<computername> account is allowed access to the file.
A. Usually the IUSR_<machine name> and IWAM_<machine name> passwords are set automatically and are never known. However, I've seen cases in which for some reason the passwords get out of sync or corrupted and need to be reset. The easiest way to reset these passwords is to extract the passwords that Microsoft IIS has in its metabase and update the accounts in Local Users and Groups to use that password.