About Windows Installer, the .NET Framework, and Visual Studio.
Microsoft Visual Studio 2008 Service Pack 1 uses a new bootstrap application that chains several packages together for a seamless installation experience. Because VS2008 SP1 contains a lot of fixes and new features, it is also quite large - almost 3 times as large as VS2005 SP1.
To download all this data, we use the Background Intelligent Transfer Service, or BITS, first and foremost. To help diagnose failures with BITS that results in falling back to other download mechanisms which may also fail, please enable BITS logging using the script StartBITSLogging.cmd available from here and attached to this post. This script enables logging to %WINDIR%\System32\bits.log. Be sure attach this file to any bug reports filed on Microsoft Connect for Visual Studio.