Fix that worked for me:
Run Steam as Administrator (does not help that you have UAC turned off, you need to run Steam as Administrator anyways)
Then start BMS again
I am running Windows 8 professional (as part of the Microsofts DreamSpark service, I have access to Windows 8 before most others)
Black Mesa does no run properly on my computer, due to the fact that the mod does not have permission/access to change/write files in it’s own directory (permissions have been updated in Win8 compared to Win7).
In the console you can read many errors regarding not beeing able to create/change config files etc. It is also not possible to save any games or configs.
Also more importantly the game crashes after the first minute during the first Loading sequence!