Recently I have had to install .NET 3.5 on a few machines with varying success. I have found that it is best to download the entire package, prior to installation – this can be found here: .NET Framework 3.5 full package.
Upon installation I found that the package, on certain machines on Windows XP, hangs on the following screen:
Now although this seems a little strange, I found that removing the network cable, therefore disconnection from the internet, kicked the internet back into life. Before doing so, wait for the installation to hang, this can be done by keeping an eye in task manager and when all activity stops, just disconnect any connection to the network/internet and then reconnect when the next screen appears.