1. Click on 'Start' and then click on 'Run.'
2. Type 'cmd' or 'command' in the box and click on 'OK.'

3. Type each of these commands and press 'Enter' after every one:
cd %windir%\system32
ren msi.dll msi.old
ren msiexec.exe msiexec.old
ren msihnd.dll msihnd.old
exit
4. Type 'exit' at the command prompt to close the Command utility.
5. Restart your computer.
6. Log on and open your Web browser and type 'www.microsoft.com' in the address bar.
7. At the Microsoft website, run your mouse-pointer over 'Downloads and Trials,' and a drop-down menu will appear. Click on 'Download Center.'
8. On the Microsoft Download Center screen, click on 'Product Families,' and a drop-down menu will appear.
9. Click on 'Windows.'
10. Click on the box that says 'Show Downloads For,' and a drop-down menu will appear.
11. Find 'Microsoft Windows Installer' on the list, click on it and then click on 'OK.'
12. Click on the most recent Microsoft Windows Installer download.
13. Download the files and save them all to your computer. Accept any terms of agreement (if you don't, the program won't install).
14. After the installation, close all open windows and restart your computer so the changes can take effect.
15. Log in, and Windows Installer should be fully functional.