There is an AI fix for a bug that can cause all of the AI
to go into notarget mode (meaning that the bug causes
everyone ingame to ignore you completely, including the
enemies).

The following is a fix that seems to work.

FIXING THE AI BUG:

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

Save the file and you're done!