The .NET Framework 3.5 runtime file package includes everything you need to run applications developed using Microsoft's .NET Framework programming tools (eg Visual Studio 2008). What does this mean for the average user of a computer? If a software author uses .NET Framework to write programs, then you will need this runtime package in order to install and use the software. Unfortunately, it can be difficult to know whether a program needs these files because often an author fails to mention it in the help files or documentation. Typically the way you find out is that you run the program and an error message appears 'This program requires .NET Framework', 'Runtime error' (caution, this error has lots of different causes) or '1645 Error Installing Microsoft .NET Framework, Return Code [followed by four numbers].' You may even get the wonderfully frustrating 'Setup Failure - unknown reason.'

With a little luck, this latest version of the .NET Framework runtime files should get you up and running. Microsoft advise that anyone with an earlier version (eg 1.0, 2.0, 3.0) should also upgrade.

According to Microsoft: '.NET Framework 3.5 contains many new features building incrementally upon .NET Framework 2.0 and 3.0, and includes .NET Framework 2.0 service pack 1 and .NET Framework 3.0 service pack 1.' In other words, this download will update any previous release. The one downside is that you will need to be online while it checks what various bits and pieces of .NEt Framework needs to be updated.