Have a look in the install log and see if there is anything obvious there:
Download Microsoft Visual Studio and .NET Framework Log Collection Tool from Official Microsoft Download Center[
^] will put a CAB file into your Temp folder containing teh install-so-far info. It may be of use.
If it isn't, then your best bet is to raise this with MS themselves - I know that they had a problem like this about a year ago, but that was fixed pretty quickly.
Their tech support isn't brilliant (quite the reverse generally) but you may get lucky! :laugh: