Try http://blogs.msdn.com/b/rpomeroy/archive/2008/06/10/how-to-manually-clean-up-a-bad-silverlight-installation.aspx

I followed all removal instructions but the '1603'-error recurred. I'm not able ro remove all references to Silverlight from the Registry though the key in HKLM\ Microsoft as per instruction, was removable. I do not understand the proposed solution to start with: Why did '1603' occur in the first place? When it was installed for the first time there were no references to or places where Silverlight was present.

You get the 1603 error because there is data left on your computer after uninstalling Silverlight. If you still have the problem after running the MS fix program then you need to remove the mentioned registry keys yourself because the presence of those keys is probably causing it.

When I try to open the subkeys of Silverlight-key HKLM\SOFTWARE\Classes\CLSID\ I get the error message: " Control cannot be opened. An error is preventing this key from being opened. Details: Access is denied". I have full administrative rights on my machine. If I try to delete the whole key I get the error message: "Cannot delete { [ the above mentioned key] }:Error while deleting key." Another key that mentions Silverlight is HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Installer, with dozens of Silverlight-keys in 'Assemblies' that all have the name 'HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Installer\Assemblies\C:|Program Files|Microsoft Silverlight|2.0.31005.0|' [and 2 other versions of Silverlight] with an 2-character language indicator and a name of a DLL file appended. However, I removed that Silverlight-folder in C:\ and subsequently ran various registry cleaners. After that there are some 20 locations in HKLM where Silverlight is also mentioned, and 2 in HKU.

If you can't delete that registry key then you must change the security settings for that key via the right-click context menu and make sure that you have write access.

I had a turned-off UAC but I still get the "Cannot delete { the above mentioned key }:Error while deleting key."

It is not about UAC, but about having write access to a specific registry key. If only the system has that permission then you won't be able to remove such a key and in that case you need to change the Permissions to that key and give yourself write permission. You can only make those changes as an Administrator.

See also http://support.microsoft.com/kb/310426 - How to set or change registry editing permissions in Windows XP or in Windows Server 2003 (Should be working in Vista as well)

I turned UAC back on; I have and had full control as an owner over the previously mentioned key and 'Administrator' and 'Administrators' have full control too. It makes no difference; I still get an 'cannot delete - error while deleting key'-message.