When you attempt to open Microsoft Office 2008, you get an error that says:

Microsoft Office 2008 is already in use

This error means that somebody else on the network is running Microsoft Office 2008 with the same product ID. Each copy of Office 2008 that is running on a network must have a different product ID. To resolve this issue, you must remove the licensing information from at least one of the computers and replace it with new information.

1. Disconnect the computer from the network.

2. Open the hard disk where Mac OS X is installed, and then open Applications.

3. Open Microsoft Office 2008, double-click Additional Tools, and then double-click Remove Office.

4. Double-click Remove Office, press and hold down the Option key, click Remove Licensing Information Only, and then click Remove.

Note: The Continue command label changes to Remove Licensing Information Only when you press the Option key.

5. Close the Remove Office window, and then click Quit.

6. Start Office 2008. When prompted, enter the correct licensing information.

If you remove Office 2008 for Mac from the computer and the issue is not resolved, follow these steps:

1. Disconnect the computer from the network.

2. Close all Microsoft Office for Mac programs.

3. On the Finder menu, click File > Find.

4. In the first criterion, select Name and Contains, and then type OfficePID.

5. In the second criterion, select Visibility, and then select Invisible items.

Note: To see the Visibility option, you may have to click Other.

6. Delete the following files:

/Library/Preferences/Microsoft/Office 2008/Microsoft Office 2008 Settings.plist
HD/Applications/Microsoft Office 2008/Office/OfficePID.plist

Note: You cannot use Spotlight in OSX 10.4.x to locate this file. You must use the Remove Office tool, and then reinstall the software.

7. Empty the trash.

8. Start Microsoft Office. When prompted, enter a new license key.

Source: Error message when you try to start an Office 2008 for Mac program: “Microsoft Office 2008 is already in use”