From Microsoft Help/Support (It seems to be working for me so far):
Step 1: Download Microsoft Windows Installer CleanUp utility
1. Visit the following link to download the utility:
2. Save the file to the computer.
3. Install this tool on the computer.
Step 2: Remove MSXML software
1. Click Start -> (All) Programs -> Windows Installer Clean Up.
2. See if you can find MSXML software. If so, remove them.
3. Restart the computer.
After that, please try to reinstall MSXML 4.0 Service Pack 2 and MSXML 4.0
SP2 Security Update KB925672 and KB927978.
Step 3: Reinstall MSXML 4.0 Service Pack 2
1. Please download MSXML 4.0 Service Pack 2 and the related updates from the
following links and save them to the Desktop.
MSXML 4.0 Service Pack 2
MSXML 4.0 SP2 Security Update (KB936181)
2. When the File Download window appears, please click the Save button, and
follow the directions to save them to the Desktop.
3. After downloading the MSXML 4.0 Service Pack 2 setup file and its updates
on to the Desktop, please double-click the files to manually install them one