critical stops can be caused by a myriad of things. Just means there was AN error.. to fix the error you have to find out why it crashed.
check eventvwr (although worthless in general every great once in awhile it shows something

) start --> run and type 'eventvwr' look at the system log and application log around the time the critical error happened.
Make sure to check for any message boxes that come up when they do occur.
Generally when gaming its a safe bet to kill antivirus (although thinking while running half life that may not be suggestable with all the exploits available in the game)
but check the TSR's to see what you are comfortable with killing during gaming.