Programs Which Interfere with Steam

If your HL² crashes on a regular bases then it could be that other programs wich are running on your pc causing the problem.
Below the 'Steam Background Applications Test'. It checkes if their are any programs running on your pc wich are know to cause trouble.

Run the Steam Background Applications Test

steam://support/?Issues=Background*

Any applications running in the background will affect gameplay and video performance. The applications listed below are known to generate connection errors or crashes, however, it is recommended that you close all background applications when playing through Steam to ensure the best performance.

Please see the How to Disable Background Applications article if you would like instructions to prevent background applications from loading when Windows starts.

Issues related to any of the programs listed below may be resolved by exiting the program and then exiting and restarting Steam.

• Antivirus Software

  Incorrectly-configured antivirus software may prevent proper operation of Steam and Steam games. VIGuard antivirus software has been found to conflict with Steam games - an error message stating "Failed to duplicate pipe handle" will be generated when attempting to load the game. Add the contents of your Steam installation folder to VIGuard's authorized folder exceptions list to correct this issue.

• Spyware Detection Software

  Steam is not spyware, however, aggressive spyware detection software may block the normal operation of Steam. We have received reports that Spycatcher Express may prevent Steam client application updates.

• Peer-to-Peer (P2P)

  P2P programs such as Kazaa, LimeWire, BitTorrent, E-mule, and Exeem can consume a large portion of your bandwidth, which may prevent Steam from connecting.

• FTP and Web Server Applications

  As with P2P programs, these programs are generally very bandwidth-intensive and may generate errors in Steam.

• IP Filtering Programs

  Peer Guardian, CYBERsitter, and Netlimiter have been found to cause Steam connection issues.

• IP Masking Programs

  Programs designed to mask your IP (GhostSurf 2007, Anonymizer, etc) will impact Steam's ability to send and receive data.

• Windows Theme Editors

  Tuneup WinStyler Theme Service has been found to cause a crash with the Steam client application and WindowBlinds may cause crashes with Source games.

• Virtual Private Network (VPN) Software

  VPN software will prevent the Steam client application from accessing the Steam network.

• Other Programs

  • Look'n'Stop has been reported to cause issues with Steam when its "Unknown UDP Packet" filter is enabled.
  • PowerStrip custom display modes software may cause issues with load time and crashing in games - it is recommended that you select the "Restore to default" setting in PowerStrip before playing (you may resume using PowerStrip normally once you are done).

Source: Steam Support