Open your registry and find the key below for your version.
Office 2000
[HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\ Uninstall\{00000409-78E1-11D2-B60F-006097C998E7}]
Office XP
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\ Uninstall\{90280409-6000-11D3-8CFE-0050048383C9}]
Change the value of "RegCompany" and "RegOwner" to the required registered company and user names.
Then find the following registry key for your Office version.
Office 2000
[HKEY_CURRENT_USER\Software\Microsoft\Office\9.0\Common]
Office XP
[HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Common]
And rename the sub-key called "UserInfo" to "OldUserInfo".
Exit your registry, close all the Office programs and restart one. The registered company should now be changed on both the splash screen and in the About dialog box.