See

http://support.microsoft.com/default.aspx?scid=kb;en-us;915430&sd=rss&spid=8291

for more detail.

The other problem reported by Isaac

http://blogs.msdn.com/junfeng/archive/2005/12/13/503059.aspx#546000

[b]
If you add the section
   <runtime>
       <developmentMode developerInstallation="true"/>
   </runtime>
to machine.config but you don't define any DEVPATH environment variable, your EXE will crash.

is not fixed by this QFE. It is a different problem and may be fixed by a separate QFE.