What windows do you use? You might need to get a service pack and do a Windows update.
Also can you confirm if you tried installation .net Framework 4.5 from here? http://www.microsoft.com/net/download
And please confirm if before installing .Net framework you uninstalled all the previous versions? If you go to Control Program - Uninstall Programs - it might show numerous versions of the .Net framework there, and you need to uninstall all of them.
If you tried all that, try installing the previous version of .NET framework http://www.microsoft.com/net/download
Look under Previous .Net version at the bottom.