I have just installed Visual Studio 2005 (with all updates) on a fresh
install of Windows 2000 (with all updates). Opened up an existing
workspace (created with VC6), and tried to compile. Nothing compiles
.... lots of C1902 errors.

This is straight out of the box - trying to compile an existing VC6
workspace.  This was my test case - to see if moving forward from VC6
to the current Visual Studio was worth doing, and how much grief we
were in for.  Not an encouraging start. :)

I see a fair number of references to this error on the web, but the
seem to apply to pre-release versions of Visual Studio, or long since
fixed bugs.

Picked the simplest project (a single .CPP file that compiles to a
command line executable, no DLLs or libraries) - as an example.  The
build output and build log follows.

------ Build started: Project: dumpwks, Configuration: Debug Win32
------
Compiling...
cl : Command line warning D9028 : minimal rebuild failure, reverting to
normal build
dumpwks.cpp
c:\asg\rwpublisher\dumpwks\dumpwks.cpp : fatal error C1902: Program
database manager mismatch; please check your installation
Build log was saved at
"file://c:\asg\RWPublisher\dumpwks\o\debug\BuildLog.htm"
dumpwks - 1 error(s), 1 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped
==========

Build Log

Build started: Project: dumpwks, Configuration: Debug|Win32

Command Lines

Creating temporary file
"c:\asg\RWPublisher\dumpwks\o\debug\RSP0000138841172.rsp" with contents
[
/Od /D "WIN32" /D "_DEBUG" /D "_CONSOLE" /D "_VC80_UPGRADE=0x0600" /D
"_MBCS" /Gm /EHsc /RTC1 /MTd /Fp".\o\debug/dumpwks.pch" /Fo".\o\debug/"
/Fd".\o\debug/" /W3 /WX /c /ZI /TP .\dumpwks.cpp
]
Creating command line "cl.exe
@c:\asg\RWPublisher\dumpwks\o\debug\RSP0000138841172.rsp /nologo
/errorReport:prompt"

Output Window

Compiling...
cl : Command line warning D9028 : minimal rebuild failure, reverting to
normal build
dumpwks.cpp
c:\asg\rwpublisher\dumpwks\dumpwks.cpp : fatal error C1902: Program
database manager mismatch; please check your installation

Results

Build log was saved at
"file://c:\asg\RWPublisher\dumpwks\o\debug\BuildLog.htm"
dumpwks - 1 error(s), 1 warning(s)