This is a possiblilty:
This problem may occur if the Setup program does not delete all the files in the WinDir\System32\Catroot2 folder during the removal of the earlier version of Windows.
Note WinDir is the Windows installation folder.
To resolve this problem, rename the %WinDir%\System32\Catroot2 folder. For more information about how to rename folders by using the Windows XP user interface, click Start, click Help and Support, type renaming folders, press ENTER, and then click Change the name of a file or folder.
To rename the %WinDir%\System32\Catroot2 folder at a command prompt, follow these steps: 1. While the Setup program is running, and while the error message still appears on the screen, press SHIFT+F10 to start a command prompt.
2. At the command prompt, type cd\windows\system32\, and then press ENTER.
3. Type rename catroot2 catold, press ENTER, and then restart the computer.
4. Try again to finish the upgrade.