I did answer this before and apologies for the answer:
The link I gave click here suggests a number of solutions. One requires you to update the windows installer program: if you have windows 98 / Me, then you need a file called InstmsiA.exe which can be found at click here
If you have windows 200 / XP, then you need a file called InstmsiW.exe which can be found at click here
Download the relevant program and run it. That may fix your problem.
Post back here if not.