As we get closer and closer to shipping Visual Studio 2005, we're making some tough decisions on what problems we can fix and what problems we'll punt and fix in Orcas.
One such problem is when mscoree.dll is hard locked for write when you try installing the 2.0 Framework. To get into this situation, you will need the 1.0 or 1.1 framework already installed on your machine and mscoree.dll needs to be hard locked for write by a third party app (could be a virus checker, etc).
If this happens, installing will show about 30 error messages and then at the end of setup, setup incorrectly says it succeeds. The problem is that a setup custom action fails so early that windows installer doesn't get a return code, assumes the process completed, and then continues on. From a customer perspective, running any managed application will fail.
The workaround is to reboot and repair the framework and everything will be fine.
We had several folks who wanted to fix this and several who felt this is such a corner case. Looking at our beta 2 data, 2 customers out of 250,000 ran into mscoree.dll hard locked for write during their install. With that data in hand, we made the tough call to postpone this bug and fix it in Orcas.
Note - this is handled correctly if the file is hard locked for read or hard locked for read/write (the more common case).