1. First turn off your firewall or virus software if you have one before downloading.
2. Open a dos box (Start -> Run -> cmd)
3. Type in
msiexec /unregister (press enter)
msiexec /regserver (press enter)
Now try installing the program (without firewall or virus programs turned on or running)
This could be caused by files in your TEMP directory. During the installation process, Windows temporarily puts installation files in this directory. At the end of the installation process, Windows is supposed to delete them, but sometimes it does not. If there are files left in there from another installation, it could interfere with the installation. Try deleting all files and subfolders in your TEMP directory, and then retry the installation.
The TEMP directory is usually located at:
C:\Documents and Settings\username\Local Settings\Temp