When you hit the classic 503 error in IIS 7 on Windows 7, the first smoking gun is always in the Application Pool. Unlike other HTTP errors, this one literally stops your pool dead in its tracks. Let's verify this with PowerShell:

Get-EventLog -LogName System -Source "WAS" -After (Get-Date).AddMinutes(-5) | 
Where-Object {$_.EventID -eq 5057} | 
Format-List -Property *

Nine times out of ten, this is an identity permission issue. The application pool tries to run under the ApplicationPoolIdentity but lacks necessary permissions. Here's the nuclear option that works when nothing else does:

icacls "C:\inetpub\wwwroot" /grant "IIS AppPool\DefaultAppPool":(OI)(CI)(RX)
icacls "%windir%\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files" /grant "IIS AppPool\DefaultAppPool":(OI)(CI)(RX)

If the basic permissions fix fails, we need to dig deeper. Create a test batch file with this content:

@echo off
echo Testing pool identity...
whoami
timeout 30

Then configure your app pool to run this as a startup script. Check the output in the IIS log directory to verify the actual execution context.

When all else fails, enable failed request tracing. Add this to your web.config:

<system.webServer>
  <tracing>
    <traceFailedRequests>
      <add path="*">
        <traceAreas>
          <add provider="ASP" verbosity="Verbose" />
          <add provider="ISAPI Extension" verbosity="Verbose" />
          <add provider="WWW Server" verbosity="Verbose" />
        </traceAreas>
        <failureDefinitions statusCodes="503" />
      </add>
    </traceFailedRequests>
  </tracing>
</system.webServer>

Windows 7 has a rarely documented limitation where certain IIS features conflict with older .NET versions. Try this registry tweak if you're using .NET 2.0/3.5:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\ASP.NET_64]
"Type"=dword:00000020

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\ASP.NET]
"Type"=dword:00000020

---

When attempting to access http://localhost/ on a Windows 7 machine with IIS installed, the application pool immediately stops, resulting in:

HTTP Error 503. The service is unavailable.

The application pool shows "Stopped" state in IIS Manager right after the access attempt.

1. Event Viewer Logs: Check Windows Event Viewer under Windows Logs > Application for W3SVC errors. Typical error pattern:

Event ID: 5002
Source: Microsoft-Windows-IIS-W3SVC-WP
Description: Application pool 'DefaultAppPool' is being automatically disabled...

2. Common Culprits:

• Incorrect .NET Framework version mapping
• Permission issues with IIS_IUSRS group
• 32-bit vs 64-bit worker process mismatch
• Missing Windows Process Activation Service (WAS)

Solution A: Reset Application Pool Identity

1. Open IIS Manager
2. Navigate to Application Pools
3. Right-click your pool → Advanced Settings
4. Under Process Model, set Identity to ApplicationPoolIdentity
5. Run in Command Prompt as Admin:

icacls "C:\inetpub\temp\appPools" /grant "IIS AppPool\DefaultAppPool":(F)

Solution B: Framework Version Alignment

%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_regiis.exe -i

Then verify in IIS:

Application Pool → .NET CLR Version → v4.0
Managed Pipeline Mode → Integrated

Enable failed request tracing:

1. IIS Manager → Sites → Default Web Site
2. Right-click → Failed Request Tracing → Enable
3. Set trace conditions (500-600 status codes)
4. Reproduce error and check trace logs at:
   %SystemDrive%\inetpub\logs\FailedReqLogFiles

For batch setups, use this script to reset permissions:

Import-Module WebAdministration
$pool = Get-Item IIS:\AppPools\DefaultAppPool
$pool.processModel.identityType = 4 # ApplicationPoolIdentity
$pool | Set-Item
Reset-WebAppPoolState -Name "DefaultAppPool"

Verify WAS configuration:

reg query HKLM\SYSTEM\CurrentControlSet\Services\WAS /v ImagePath

Should return:

%SystemRoot%\system32\svchost.exe -k iissvcs