if you are using Windows 8 RTM or any early build then you might have found an error while you are trying to run or install any software which requires .Net Framework 3.5 then you might get an error which say you need to install .Net Framework 3.5.
Windows 8 comes with .Net Framework 4.5 but still it ask you to install .Net Framework 3.5, Actually Windows 8 comes with .Net Framework 3.5 but by default its not enabled. So here we are going to activate it.
Now mount or insert your Windows 8 disk and note the Drive letter of your mounted or DVD ROM/RAM, Now Press Windows Key + X on your keyboard and you will get a small menu, Click on Command Prompt (Admin).
Now Command Prompt will come up, provide following command to it, Replace H with your drive letter:-
dism.exe /online /enable-feature /featurename:NetFX3 /Source:H:sourcessxs /LimitAccess
It will take some time but after process is completed you will be able to run all apps.