how do i reinstall windows installer

Check your hard disk for any errors:

Click Start

Type: CMD, from the results, right click CMD

Click 'Run as Administrator'

At the Command Prompt, type: chkdsk /r /f

When you restart your system, your computer will be scanned for errors and attempts will be made to correct them.

then

Click Start
Type: CMD, from the results, right click CMD
Click 'Run as Administrator'
At the Command Prompt, type: sfc/scannow

This will check for any integrity violations

Restart your system

also review:

Troubleshoot Windows Installer

2438651 How to troubleshoot problems that may occur when you install, uninstall, or upgrade a program on a Windows-based computer

http://support.microsoft.com/kb/2438651

Reinstall Windows Installer Service

Have you recently installed any applications or drivers that might have interfared with the service? You could try removing them temporarily to see if it corrects the problem. Have you also tried doing a System Restore to a point in time before this problem started happening?

Also try this suggest by Karen Ji, MS Forum Moderator for reinstalling the Windows Installer service:

To reinstall the Windows Installer, rename the damaged Windows Installer files, and then reinstall the Windows Installer. To do this, follow these steps:

1. Click Start, click Run, type cmd in the Open text box, and then click OK.

2. Type cd %windir%\system32, and then press ENTER.

3. Type attrib -r -s -h dllcache, and then press ENTER.

4. Type ren msi.dll msi.old, and then press ENTER.

5. Type ren msiexec.exe msiexec.old, and then press ENTER.

6. Type ren msihnd.dll msihnd.old, and then press ENTER.

7. Type exit, and then press ENTER.

8. At the command prompt, type exit, and then press ENTER.

9. Click Start, and then click Turn off computer or Shut Down.

10. Select the Restart option, and then click OK, or click Restart.

11. Log on to the computer.

12. Download and install the latest version of the Windows Installer from the link below

http://www.microsoft.com/downloads/details.aspx?FamilyID=5a58b56f-60b6-4412-95b9-54d056d6f9f4&displaylang=en