Recently Microsoft released their .NET framework 4.0 where as .NET framework 3.5 released long back in 2007.As all know to run certain applications developed on .NET framework users need to install .NET framework in their computers.Installing .NET framework 4.0 online is a long process hence you can try useful .NET framework 4.0 offline installer.
If you want to know the version of .NET framework installed in your Computer
Read 2 Ways to Find Version of .Net Framework Installed in Your Computer
.Net Framework Version Checker 1.0.0.0
If .Net framework installation fails you can use cleanup tool to repair and uninstall those installations.
You need administrative privileges to run .Net framework standalone installer .
Download .Net framework 4.0 offline installer, file size is 48 MB.