In VS 2005 and 2008 (but not in VS.NET 2002 or 2003), you could press the Shift key while VS was being launched to prevent the loading of add-ins that were marked to load on startup. This was very useful for developers of add-ins to prevent the following issue:

PRB: ‘Could not copy temporary files to the output directory’ error building Visual Studio .NET add-in.
http://www.mztools.com/articles/2005/MZ2005013.aspx

On Windows 7 and its new bar to launch/show running applications, the Shift key is used to launch a new application, so Microsoft removed that behavior of VS 2010. I reported it when I noticed it in the Jan LCTP build. The report is here:

VSIP: VS 2010 Jan LCTP: Shift key doesn’t prevent loading add-ins when VS is launched
https://connect.microsoft.com/VisualStudio/feedback/details/526271

The workaround is to use the /SafeMode command-line switch, I updated the article above.