Said it before with your other issue, you need to reinstall Windows. Maybe power users see it more often but after two years my computers get OS rot bad enough that I can't take it any longer.
Other options to try first:
1) Download CCleaner run it, then run it's registry cleaner.
2) Download Glarysoft Glary Utilities (free version) and do the same thing.
Then reboot and try to install NET Framework again. FYI, there's a standalone download for NET Framework 4.5.1 that includes previous versions.http://www.microsoft.com/en-us/download ... x?id=40779