Try doing a repair install, then try the update again:
Method 1 – Open Word, go to Help – Select Detect and Repair
or use Method 2 -
1. Click Start, and then click Control Panel.
2. Click Add or Remove Programs.
3. Click the Office program that you want to repair, and then click Change.
4. In the Maintenance Mode Options dialog box, click Reinstall or Repair,
and then
click Next.
5. In the Reinstall or Repair Office dialog box, click Detect and Repair
errors in my
Office installation, and then click Install.
6. When the repair process is finished, run the update again.
Most likely you will need to have your Office CD handy.