This is from the support pages at www.steampowered.com.  There are many
support articles and a fairly easy-to-use search utility.

             Question
             Crashing and Lock ups with Source-Engine Games

             Answer
             Hello,

           We are sorry for any problems you may have experienced due
recent Source-Engine updates. Below are known issues and fixes we have found
so far:

           If you try and start the game and end up at your desktop you
should download and install the latest version of directx 9.0c found here:

           http://www.microsoft.com/downloads/search.aspx?displaylang=en&categoryid=2

           If you are locking up at the Loading screen you need to manually
re-add the hl2.exe for each of your Steam games to your exceptions list.
Once the game exe has been updated on your Firewall you should no longer
lock up at the Loading screen.

           On machines using nForce audio go to Options > Audio and switch
to 2 speakers. This should stop you from crashing when joining your game.

           There is also a crash that can happen with nVidia based video
cards that is corrected with the following Beta drivers from nVidia:

           http://www.nzone.com/object/nzone_downloads_winxp_2k_78.05.html

           If you are still having crashes while playing your game please
try the following steps:

           Clean booting to free up memory on machines that have 512mb of
ram.

           When you start Windows, typically there are a number of programs
that start automatically and run in the background that may interfere with
the game. These programs may include antivirus and system utility programs.
When you perform a clean boot, you prevent these programs from starting
automatically. You must be logged on as an administrator or a member of the
Administrators group to complete this procedure.

             1.. Click Start, and then click Run.
             2.. In the Open box, type msconfig, and then click OK.
             3.. On the General tab, click Selective Startup.
             4.. Under Selective Startup, click to clear the following
check boxes:
               a.. Process SYSTEM.INI File
               b.. Process WIN.INI File
               c.. Load Startup Items
             5.. On the Services tab, click to select the Hide All
Microsoft Services check box, and then click Disable All.
             6.. Click OK, and then click Restart.
           Note: To restore your normal boot sequence re-open msconfig and
place a check in the items you previously disabled.

           For lag issues in game or if you're still not able to get to the
main menu try specifying a different DXLevel in launch options. This update
replaced any tweaks that may have been made to improve game performance. If
you are crashing or locking up when starting your game go to the Launch
Options for the game and add -dxlevel 81.

           If you are able to play with -window - console, but crash
without these two launch options try copying the avi.dll from:

           Steam\SteamApps\accountname\counter-strike source\bin folder to
the Steam\SteamApps\accountname\counter-strike source folder

           This will also work for HL2 and DoD: S.

           If after trying these steps you are still experiencing issues
please re-open your ticket and update us with the steps you have taken so
far.

           -also-

                         Question
                         When I start my game I lock up at the Loading
Screen.

                         Answer
                         For those using Firewall software, you need to
manually re-add C:\Program Files\Valve\Steam\SteamApps\username\half-life
2\hl2.exe to your exceptions list. Once the game exe has been updated on
your Firewall you should no longer lock up at the Loading screen.

                       If you continue to have issues please click Ask a
Question at the top of this page and submit a support ticket.

A result, get a new computer because it may be a piece of crap like some
PCs I've seen in my day.  Reply and give me the sys_stats of your
computer system