How do I properly upgrade to .Net 4.0 from 2.x

  My goal is to get AJAX into Sharepoint and found the the .NET Framework is at 2. So I thought it necessary to update to .NET 4.0.  I did a windows update/install and it installed the 4.0. However when I switch(using IIS Manager) from one of the Sharepoint sites from 2 to 4 Sharepoint gives the HTTP Error 404. So I have to switch it back to 2 in order for the site to remain up.  What have I done incorrectly?

While I don't think this has anything to do with it...its worth mentioning.
I added AJAX dll to Sharepoints BIN directory.

Added the following to the web.config.
    <add assembly="AjaxControlToolkit, Version=, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e"/>
    <add namespace="AjaxControlToolkit" assembly="AjaxControlToolkit" tagPrefix="ajaxToolkit"/>

Please give detailed instructions.
Who is Participating?
Jimmy AndrewsConnect With a Mentor Founder, What2do.LiveCommented:
Take a look here, I think this is what you are looking for, it is fairly detailed as well.

LIFEEXPERTAuthor Commented:
Project was canceled... and did investigate further
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