I have received several errors with MiKTeX 2.9, being portable or standard install, in which some files related to configuration, does not work well.
In my laptop (Win XP, spanish), the user's configuration directory, has an accented character. Example: ..\djnavas\Configuración local\Datos de programa\MiXTeX\
and the standard install created a directory with the name changed rewriting the accented character, like in "Configuraci%A5n local" (not necessarily equal as in my machine).
In my office, I use portable MiKTeX from an USB. The program works, but have the difficulty that can't rebuild the formats after pressing the button "Update Formats".
The error report I copied, is:
MiKTeX Problem Report Message: Windows
API error 5: Acceso denegado.Data: Source:
Libraries\MiKTeX\Core\win\winProcess.cpp
Line: 810
Acceso denegado means, "access denied".
This problem happened after an update of packages from a mirror, through the web.
I hope to read some ideas of what to do.
Best Answer
There has been a bug report with an "windows API 5" error. The comments say that the update of today (15.12.2010) of miktex-runtime-bin has solved the problem. (In case the update isn't yet in your mirror: Christian wrote that he has set up a temporary mirror ctan.miktex.org where you can get the update.)