Really odd that you don't have the IIS_IUSRS. I'm on a Windows Server 2008 R2 Standard (Full not Core) myself, but if i remember correctly IIS_IUSRS did exist in the first release of Server 2008 as well, perhaps you tried finding the user account by inputting only partial name? I have had the same issues of Windows telling me a user account doesn't exist, but when full name has been written it gets found after all.
As for the 500 error, check the status of your PHP installation setup/config. My guess would be that PHP is crashing FastCGI and that may be the reason why you can't access install.php. If installation set up /config looks allright you could check the IIS/PHP logs(can't recall the exact location where they were stored). Have you even have Zend Server installed for PHP application development by any chance or it's a standard PHP release for Windows?
On another sidenote, WordPress is compatible being set up in a "Network Mode" in which you can have 1 physical WP folder set up, but have multiple WP websites run on it at the same time .... so instead of having 4 folders for 4 websites and needing to set up 4 admin accounts in each website, you could have 4 virtual WP websites controled trough 1 master admin dashboard where you would be able to create new WP Blog websites and assign their corresponding editors user accounts to them .. trough this you could also manage all the plugins/themes available to individual WP Websites while at the same time having all the plugins stored in the same single physical folder located on your HDD. This is an advanced WP configuration which can ease up the folder permission assignation for each WP website. If that would work better for you, you'll be able to find a installation guide somewhere on WordPress website explaining how to set up WordPress in a Network mode.
I also really doubt that this might be a .htaccess or a plugin issue since it's a fresh installation.