If you're having troubles installing .NET 3.5 on to Windows 8, here's a quick tip that may solve your problem.
Make sure your Windows 8 installer disc is in your drive (or mounted/copied somewhere), then from an elevated command prompt run this command:
dism.exe /online /enable-feature /featurename:NetFX3 /Source:d:\\sources\\sxs /LimitAccess
Replace d:\ with the drive/path to your local sources.
As a sidenote, I needed .NET 3.5 for SQL Server 2012.