IIS ApplicationPoolIdentity 503 error

I've recently ported to a different hosting company (VDS), and on the new server, installed IIS 7.5 Express, latest versions of PHP and MySQL.

I then copied the files from the old server to new server (NOTHING beyond the content of the folders of my websites, so no config files or anything except the web.configs in the sites themselves), set-up websites. I've tried to visit a website, and I got a 503 Service Unavailable message, tried both remotely and locally at the server, no change.

Tried on every site, even on Default Web Site, and no change: still 503. I've checked the permissions to the folders of the websites (and all the children), and both my account, Network Service, Administrators have effectively full control of all the contents.

All the apps run in DefaultAppPool, which runs under ApplicationPoolIdentity. Tried creating a new app pool, moving sites there, no luck, tried setting "Load User Profile" to False in pool settings (read it here HTTP Error 503 on IIS 7.5 after SP Install), no change.

The event viewer in an extremely detailed way says:

"A process serving application pool 'DefaultAppPool' terminated unexpectedly. The process id was '2356'. The process exit code was '0xfffffffe'."

and nothing else. Five times of this error, and the pool is shut down due to rapid protection. I restart the pool, try again, nothing changes. No matter I'm trying to get an .aspx, .php, or static content. Here is the most weird part: when I set the app pool to run under LocalSystem, it works. No problem. But everything else fails.

I've checked the website folders about ten times, changed, erased, re-added rights etc tried everything but no change. It's as if there is another file that I'm forgetting to check that the pool's user is unable to access. But the event viewer does not help me. This started with the new server and I tried after installing everything, so I can't tell after what this started happening. I obviously won't be running under LocalSystem. Even my own user, Administrator, doesn't work.

The only identity that works is LocalSystem. My user, LocalService, NetworkService, ApplicationPoolIdentity all fail the same way. I'm going crazy, I'm 99% sure this is a user-rights issue. But all the website files are accessible, and I haven't changed anything in the system32 inetconfig or anywhere.

Ok, found my own answer again. The process w3wp.exe runs under the DefaultAppPool user, not NETWORK SERVICE. I don't know why it doesn't run under NETWORK SERVICE, but after some research it turned out that the process needed access to C:\Windows\System32\inetsrv\config\schema and giving NETWORK SERVICE access didn't change anything. I've by coincidence found somewhere about the DefaultAppPool user, gave read permissions, and by starting the pool, all my websites started working perfectly. Quite weird, I always thought that the IIS processes ran under NETWORK SERVICE by default, and I'm sure that I haven't changed any setting.