HTTP Error 500.21 - Internal Server ErrorHandler "PageHandlerFactory-Integrated" has a badmodule "ManagedPipelineHandler" in its module list

This is the error due to not installing the .net frame work 4.0 on IIS7.x server. To install the .net 4.0 frame work run the following on command prompt.
on 32-bit operating system
%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_regiis.exe –i


on 64-bit operating system
%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_regiis.exe –i

If the above solution not worked then apparently, the reason for recieving the Internal Server error message was that installed SQL Server 2008, after installing Visual Studio 2010 vice versa, and because of this it corrupted the IIS Machine level configuration files ("If you install VS2010 and then install VS2008 and VS2008 SP1, the configuration files for ASP.NET in IIS only include about 1/2 of the correct .Net 4.0 configuration sections." read more here).

To repair this problem run a full silent repair of the .NET Framework 4.0.   Here's how on either a 32 bit or 64 bit computer:

1. Click Start -> All Programs -> Accessories -> Run
2. In the Open textbox paste in the following line (see list of all .NET Framework version install, repair and unistall command lines here):
   For silent repair on 32 bit computer with .Net Framework version 4.0.30319 use:
   %windir%\Microsoft.NET\Framework\v4.0.30319\SetupCache\Client\setup.exe /repair /x86 /x64 /ia64 /parameterfolder Client /q /norestart
   
   For silent repair on 64 bit computer with .Net Framework version 4.0.30319 use:
   %windir%\Microsoft.NET\Framework64\v4.0.30319\SetupCache\Client\setup.exe /repair /x86 /x64 /ia64 /parameterfolder Client /q /norestart
   
   
3. Click OK to start the repair
4. After, the repair ran for a few minutes, I restarted IIS 7.5, and things began to work correctly!

Hopefully, this will work for you...

How to start microsoft single sign-on service

Use the following procedure to start the Single Sign-On service.

Start the Single Sign-On service

1. From Administrative Tools, click Services.
2. Double-click Microsoft Single Sign-On Service.
3. On the Log On tab of the Single Sign-On Service Properties page, click This account, and then type the domain, user name, and password that you have used to install and manage your server.
4. Click Apply.
5. On the General tab of the Single Sign-On Service Properties page, change the startup type to Automatic, click Start, and then click OK.

SharePoint – Access Denied When Enabling The Office SharePoint ServerPublishing Infrastructure,Site Collection Feature

Cause:
The application pool account does not have access to create / update timer jobs which is required when enabling the Office SharePoint Server Publishing Infrastructure Site Collection feature.
Solution:
There are two options:

1. Temporarily add the application pool account to the farm administrators group, or
2. Run the following command:
   stsadm -o activatefeature -name PublishingResources -url http://SITECOLLECTIONURL

Steps to add application pool account to the farms administrator group

1. Start > Programs > Administrative Tools > IIS Manager
2. Expand the Application Pools node, locate the application pool associated with the site collection that is experiencing an issue
3. Right-Click the Application Pool and select "Properties"
4. Select the "Identity" tab
5.  Note the user account
6. Start > Programs > Microsoft Office Server > SharePoint 3.0 Central Administration
7.  Site Actions > Site Settings > Advanced Permissions
8. Click "Groups"
9.  Click "Farm Administrators"
10. New > Add Users
11. Add Users: Type the name of the Application Pool Identity account
12. Give Permission: Add users to a SharePoint group: Farm Administrators
13. Add the Application Pool account to the Farm Administrators group
14.  Send E-Mail: Uncheck (you most likely do not want to send an email)
15.  Click OK
16. Try again to activate the "Office SharePoint Server Publishing Infrastructure" Site Collection Feature