When reinstalling Visual Studio and accessing Web projects that use Telerik RadControls for ASP.Net Ajax, I've found that Intellisense is generally broken for that project and only for Telerik components. Furthermore, design-time support is also lost. Here is a list of fixes to try in order to solve this.
Check whether the correct version of the components are in the GAC.
If the site is running on a network share, be sure that the UNC share has fully trust by the .Net framework. This can be entered at Start -> Control Panel -> Administrative Tools -> Microsoft .NET Framework 2.0 Configuration -> Console Root -> .NET Framework 2.0 Configuration -> My Computer -> Runtime Security Policy -> User -> Code Groups -> All_Code, then add a child code group with file:////\\computername\sharename\
* with Full Trust permissions.
Right-click the assembly in the /bin folder of the project and click Update Reference.