There are numerous people who have reported they cannot install .Net 3.5 SP1 on top of a .Net 2.0 installation. Typically this can happen if there has been a lot of install and uninstall activity between the two .Net installs.
Most often this happens when people upgrade Visual Studio. There are a lot of posts on some of the VS forums about this problem. (More people have VS than SQL, so most of the posts are in VS forums.) There is a tool from Microsoft that is supposed to repair the .Net 2.0 installation so you can put .Net 3.5 SP1 on top of it, but my experience is this tool does not always work.
We had this problem on our developer boxes when we upgraded to VS 2008. 90% of the boxes upgraded without any problem. About half of the remaining boxes allowed the upgrade to proceed after running the repair tool. For the rest of the boxes (including mine :() we had to re-install Windows.
Original author: https://github.com/SQL-FineBuild/Common/wiki/ 1-click install and best practice configuration of SQL Server 2019, 2017 2016, 2014, 2012, 2008 R2, 2008 and 2005.
When I give food to the poor they call me a saint. When I ask why they are poor they call me a communist - Archbishop Hélder Câmara