To install framework 3.5 you don't need to download it as it comes with windows 7 as OS component.
Follow steps mentioned below to install it.
Step 1. Click on start > control panel > Programs
Step 2. Click on turn windows features on or off, under Programs and Features section
Step 3. Select Microsoft .Net Framework 3.5.1 and click on ok
.Net framework 3.5 SP1 will now be installed.