Go into your hl2\cfg\ folder, and create a txt file, inside this text file, write the following lines.
ai_norebuildgraph "1"
ai_disable "0"
Save, then rename the text file to ai_fix.cfg, right click the file and set it to read only.
Next, open up valve.rc in notepad, you should see the following
// load the base configuration
//exec default.cfg
// Setup custom controller
exec joystick.cfg
// run a user script file if present
exec autoexec.cfg
//
// stuff command line statements
//
stuffcmds
// display the startup level
startupmenu
under "exec autoexec.cfg", add this line
exec ai_fix.cfg
