Several SIL tools like FieldWorks Language Explorer (Flex) and SayMore depend on the dotnet framework. (That’s a bit like Microsoft’s answer to Java.) Unfortunately it sometimes falls over.
So I found a rather handy blog post that explains what to do when you try installing DotNet 4.0 and it sits still for ages then responds with the less-than-helpful error message “.NET framework has not been installed because: HRESULT 0xc8000222.“
It seems that a folder gets corrupted but that you can’t remove the auto-update folder until the relevant process is stopped. So, you really want to remove the directory %windir%/SoftwareDistribution but first of all you have to do this at the command prompt (net stop WuAuServ) to stop I guess an auto-update process which uses that folder. Then remove or rename the folder %windir%/SoftwareDistribution before restarting the relevant process (net start WuAuServ). Then DotNet should install.
Finally, restart your computer after installing .NET and before installing FieldWorks or SayMore. It possibly could work without you doing so but I haven’t had much success with that.