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Hello Harper,

Thank you for contacting Steam Support.

If you are having difficulty accessing the multiplayer portion of the game due to a key error, please take the follwing steps:

-If you are running in Windows Vista or Windows 7, please completely exit Steam, and right-click on the shortcut which you use to launch Steam and select "Run as Administrator."

-If you are running in Windows XP, take the following steps:
1) Exit Steam.
2) Go to the "Start" menu, select "Run", type "regedit" and click "OK".
3) Navigate to the following registry key: HKEY_CURRENT_USER\SOFTWARE\VALVE
4) Right click and select "Permissions"
5) Check the boxes for "Admin" and "System" to "Full control" and "Read".

First, verify the GCF Cache for the game:
Title: Verifying GCF Cache Files
Link: http://support.steampowered.com/kb_article.php?ref=2037-QEUH-3335

Verifying the GCF Cache solves most CD Key issues for most users.

NOTE: The CD Key for this title is not shown to the user in Steam.

If it still does not work, please try deleting your Call of Duty 4 registry entry:

Go to the "Start" menu, select "Run", type "regedit" and click "OK".

If you are using a 32-bit version of Windows, navigate to the following registry key: HKEY_LOCAL_MACHINE\SOFTWARE\Activision\Call of Duty 4
Note: In 64-bit operating systems, this path may begin as: HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Activision\Call of Duty 4

The contents of the registry entry "codkey" is your CD Key. Please write this key down for future reference.

Right-click on the entry "codkey" and select "Delete".

Exit regedit.

Restart your computer.

Log in to your Steam account using the following instructions:

-If you are running in Windows Vista or Windows 7, right-click on the shortcut which you use to launch Steam and select "Run as Administrator."

-If you are running in Windows XP, ensure the the following steps have been taken before starting Steam:
1) Exit Steam.
2) Go to the "Start" menu, select "Run", type "regedit" and click "OK".
3) Navigate to the following registry key: HKEY_CURRENT_USER\SOFTWARE\VALVE
4) Right click and select "Permissions"
5) Check the boxes for "Admin" and "System" to "Full control" and "Read".

From the Library list, right-click on "Call of Duty 4" and select "Properties" from the menu.

Select the "Local files" tab and click the "Verify integrity of game cache..." button.

Steam will verify the game cache files (GCF's) - this process may take several minutes.

Once the process is completed, the "Check" window will automatically exit and you should be able to launch your game.

You may also try using the key previously found in your registry if COD4 asks for a CD Key again.

If you continue to have difficulty, further technical support for this title is handled by a third party support department - please follow the link below for instructions to contact the support provider for this title:

Title: Call of Duty 4: Modern Warfare
Link: http://support.steampowered.com/kb_article.php?ref=8746-WDHC-4827

Please let us know if you encounter any difficulty contacting the support provider for this title, or if you have any further questions.