Generally the contents of the toolbox are related to the .NET version of the project you have loaded. Creating a website targetting .NET 2.0 will populate toolbox items for .NET 2.0 which should be the Q3 2010 SP2 installation, since later version of the RadControls for ASP.NET AJAX do not have .NET 2.0 assemblies.
If you have installed the latest version opening a .NET 3.5 or .NET 4.0 site will have the Q2 2012 SP1 RadControls for ASP.NET AJAX populated in the toolbox. When you open a .NET 2.0 website if you do not see the old version of the RadControls you can open the toolbox, right click on it, click on Choose Items and from .NET components Browse to the .NET 2.0 Telerik.Web.UI assembly so that it is added to the toolbox.
I just tested this scenario with the version I mentioned above and things are working as expected. Just make sure the Target Framework of th site is set to .NET 2.0 in the Property Pages -> Build tab of the site.
What I would advise to facilitate this switch is that you close VisualStudio to allow it to unload the assemblies from the previous site before opening the next.
The best approach would be to upgrade all projects to the recent version of the RadControls to benefit from the fixes and features that are incorporated in them for the past year.
All the best,
the Telerik team
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