1. Press windows Start button.
2. Write cmd
4. Write dism /online /enable-feature /featurename:NetFX3 /all /Source:d:\sources\sxs /LimitAccess
Note: D: in this case is my Windows installation disc.
5 . Press enter and wait for the installation to finish.
6. Finished, .NET Framework 3.5 is now installed and you can continue to install your software.