Which version?
Gives no updates for V8.

I've win8 and is all perfect.

Hello,

Please try to uninstall WebSite X5 and then delete  the folder C:/Program Files/WebSite X5 v8.

Once done please try to remove all the version of the framework from your pc and then use the FX Cleanup tool you can find at http://cid-27e6a35d1a492af7.skydrive.live.com/self.aspx/Blog_Tools/dotnetfx_cleanup_tool.zip (alternative link: http://blogs.msdn.com/cfs-file.ashx/__key/CommunityServer-Components-PostAttachments/00-08-90-44-93/dotnetfx_5F00_cleanup_5F00_tool.zip ).
Once the tool has cleaned the system download the Framework installer from following link http://www.microsoft.com/en-us/download/details.aspx?id=25150 and install it again.
Once the .NET Framework is installed reinstall WebSite X5 as administrator with right-click - run as administrator.

Please let me know if this helps!