I've followed the following instructions from the link the error gave:
Windows Update error 80070643
If you receive Windows Update error 80070643 while installing updates for Microsoft Office 2003, you might need to restart the Office Source Engine (OSE) service. Windows Update can automatically start the service to install updates for Office products if the service is set to the default Manual setting. If the service is disabled, you need to restart it, and then try to install the latest updates for Office products again.
To restart the Office Source Engine (OSE) service
You must be logged on as an administrator to perform these steps.
Click to open Administrative Tools.
Double-click Services. If you are prompted for an administrator password or confirmation, type the password or provide confirmation.
If the Office Source Engine service is disabled, double-click it to open the service properties.
Click the Startup type list, click Automatic (Delayed Start), and then click Apply.
Under Service status, click Start.
However, it still doesn't help in the installation of the update. Anyone else have the same problem or could help me with it? Thanks!