Something wrong with (my) Steam?

Most of my Steam games seem to not be working anymore including Dogs of War (w/c I have just downloaded) and a messagebox will usually come out saying:

Steam - Error

Failed to start game (unknown error).


I've tried going to the executable (*.exe) files and trying to run them without using the shortcuts... a new message box appears saying:

Windows cannot access the specified device, path, or file.
You may not have the appropriate permissions to access the item.

Has anybody seen this before?
Can anybody please help me with this technical problem?

Thanks!
ZeroCorsair