I've searched everywhere on my computer. It is not under add and remove programs, it's not in the registry, and it's not in my program files. I know I installed it once in the past, and I wanted to upgrade it. Why can't I install it? How do I find out what's wrong?
Hummm... that strange...
As Ken mentionned if it's already installed then it should indeed be listed under installed updates
when look in add remove programs through your Control Panel.
Also if you run the regedit it should also appear
'- Microsoft SDK's
'- v 3.0
Have you checked the following link dealing with
You receive an "error 1603: A fatal error occurred during installation" error message when you try to install a Windows Installer package
or check the following thread
Maybe this can help you solve your problem.
Try checking the silverlight log file under the (for Vista) C:\users\accountname\appdata\local\temp\ directory. I had that same error, it turned out to be an access denied error on the HKLM\Software\Mozilla\Firefox\Extensions key. I have McAfee Enterprise 8.70i installed on a locked down Vista machine and I went round in circles trying to install Silverlight_tools version 4.
Turns out I had to go to Start->All Progams->McAfee, right-click on VirusScan Console, then run as administrator.
I disable Access Protection which involves watching it as it re-enables itself every 5 minutes or so. As long as I keep it disabled during the install of the various Silverlight plugins and tools, the install succeeds and Visiting this site with Internet Explorer 7 shows Silverlight 4 is installed.
I guess that even though my PC does not have FireFox installed on it, McAfee doesn't care, as its protection scheme is apparently "block all firefox extensions at all costs". Silverlight also for some unknown reason to me, is hard set to change a value under that Mozilla Firefox Extensions key, and it aborts if it cannot do so.
The one thing I got from this experience is I'm quite impressed at how much power McAfee has over my PC. I went into regedit, took ownership, gave my account full control, and cascaded the permissions throughout the Mozilla key and its subkeys, yet I could NOT change a value under the Extensions key. It always gave me an Access Denied message. Once I disabled McAfee Access Protection, I hit delete on the Mozilla key and away it went.
I hope this is helpful to someone. It blew my mind that Vista seemed to ignore my deletion requests even though I had full control AND ownership of the registry keys.
I'm sure this approach applies to other AV softwares like Norton Internet Security, etc. Try disabling your AV during install or manually manipulating the keys mentioned in the silverlight log file.