Its cause you didn't a ATI Card. Na just kidding.
I'll go look up the steam support page.
~Edit~
The steam Support page is below
~Edit 2~
Phpbb doesn't like the long link, so I'll just post what it says below
Quote:
General Troubleshooting Steps
Question
General Troubleshooting Steps or what to do when the game stops responding or quits unexpectedly.
Answer
When you start or play any Steam games, you may experience one or more of the following symptoms:
* The game stops responding.
* The game quits unexpectedly.
* The computer restarts unexpectedly.
* Sound problems
* Memory Errors when running Half-Life 2 and other Source Games
To resolve these types of issues, try the following:
Clean boot procedure for Windows XP.
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:
* Process SYSTEM.INI File
* Process WIN.INI File
* 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.
Adjust Graphics Hardware Acceleration
To adjust graphics hardware acceleration, use the method described for your operating system:
Microsoft Windows 2000 or Microsoft Windows XP
1. Click Start, point to Settings, and then click Control Panel.
2. Double-click Display.
3. On the Settings tab, click Advanced.
4. Click the Troubleshoot tab.
5. Move the Hardware Acceleration slider two notches to the left of Full (the Disable all cursor and advanced drawing accelerations setting).
6. Click OK, and then click OK again.
7. Test the game.
Microsoft Windows 98 or Microsoft Windows Millennium Edition (Me)
1. Click Start, point to Settings, and then click Control Panel.
2. Double-click System.
3. On the Performance tab, click Graphics.
4. Move the Hardware Acceleration slider one notch to the left of Full (the Most acceleration setting).
5. Click OK, and then click Close.
6. If you are prompted to restart the computer, click Yes.
Disable AGP Texturing in the DirectX Diagnostic Tool
1. Click Start, and then click Run.
2. In the Open box, type dxdiag, and then click OK.
3. Click the Display tab.
4. Under DirectX Features next to AGP Texture Acceleration, click Disable.
5. When you receive the following warning message, click OK:
This will disable usage of AGP (Acceleration Graphics Port) for all display devices on your system which support it.
6. Click Exit.
Temporarily Disable the Sound Card in Windows
This method will help you to identify if the sound card is causing the problem. If the following steps resolve this issue, contact your computer manufacturer for further troubleshooting steps. To temporarily disable your sound card, follow these steps:
1. Click Start, and then click Control Panel.
2. Double-click System.
3. Click the Hardware tab, and then click Device Manager.
For Windows Millennium Edition (Me), just click the Device Manager tab.
4. Double-click Sound, Video and Game Controllers.
5. Double-click your sound card.
6. Under Device Usage, click Do Not Use This Device (Disable).
For Windows Millennium Edition (Me), follow these additional steps:
1. On the General tab, under Device Usage, click to clear the Original Configuration (Current) check box.
2. If you do not see this check box, click to select the Disable In This Hardware Profile check box.
7. Restart Windows, and then test the game.
Lower Sound Acceleration
1. Click Start, and then click Run.
2. In the Open box, type dxdiag, and then click OK.
3. On the Sound tab, in the DirectX Features section, change the Hardware Sound Acceleration Level to Basic acceleration.
4. Click Exit.
Update Your Video Driver
Contact the manufacturer of your video adapter for information about how to obtain and install the latest version of the video driver for your video adapter.
To identify the name and manufacturer of your video adapter:
1. Click Start, and then click Run.
2. In the Open box, type dxdiag, and then click OK.
3. Click the Display tab.
4. Under Device, look for Name and Manufacturer.
5. Click Exit.
Update Your Sound Driver
Contact the manufacturer of your sound card to inquire about how to obtain and install the latest version of the sound driver for your sound.
To identify the name and provider of your sound driver:
1. Click Start, and then click Run.
2. In the Open box, type dxdiag, and then click OK.
3. Click the Sound tab.
4. Under Device, look for Name, under Driver, look for Provider.
5. Click Exit.
Scan Your Computer for Viruses and Remove Spyware
The game may quit, generate an error, or stop responding during start or during play if your computer is infected with the virus.
Important: If you already have an antivirus program installed, before you scan your computer for viruses, remember update your antivirus program with the latest signature files. Most software companies that create antivirus programs release a new signature file each month. After you obtain the latest signature files, scan your computer. For help obtaining the antivirus signature files, contact the antivus software manufacturer.
Spyware is software that collects personal information without your knowledge or permission. You might be the target of spyware or other unwanted software if you download music from file-sharing programs, free games from sites that you do not trust, or other software from an unknown source. Spyware may effect the game play as well as it's startup process.