Open the Control Panel, double click on "Add/Remove Programs", then scroll down to "Microsoft NET Framework 3.5 SP1" which is the latest for 3.5.
If you'd like to install 4.0 on the machine, you'll probably want to have 3.5 SP1 install previously.. If you've already got it installed, plus all of its updates from Windows Updates), then download both of the 4.0 manual installers from the links below, then install them in the order I list them.. First the "4.0 Client Profile", then the actual 4.0.
Microsoft .NET Framework 4 Client Profile (Standalone Installer)
Microsoft .NET Framework 4 (Standalone Installer)
Hope this helps.
I use XP SP2. I think I have Net framework 3.5. How do I find out for sure? Where can I look on my computer to find out? I see more and more software wanting 4.0. How do I upgrade to 4.0?