check in the setting of the game as of wat is the Direct X renderer!!! is it 8 or 9??? and then search in the folder of where you installed HL:e1, there u can find files with names containing .dx9 or .dx8 ... If the files are .dx9 and the renderer in the in game settings is dx 8,, (backup/ or be ready to reinstall, jst in case!!!) then rename all the files with dx9 to dx8!!!
ALTERNATIVELY, to make sure its working,, take the in game console by pressing ~ ,,,, There may be error messages of which all texture files are missing,,, search those file names in your installed folder,, then rename according to what i said before..
Then check the in game console whether the same error message is coming in the same area!!!
Hope you understood!!!!