Vista - Why does .NET v3.5-SP1 fail? v3.0 required? MSI return code 1603

I tried to install .NET v3.5+SP1 on my WinXP+SP3. Unfortunately it fails.

Why?

The log window shows the following error (ignore line wrapping):

[11/07/09,17:33:10] Microsoft .NET Framework 3.0 SP2 x86: [2] Error: Installation failed for component Microsoft .NET Framework 3.0 SP2 x86. MSI returned error code 1603
[11/07/09,17:33:16] WapUI: [2] DepCheck indicates Microsoft .NET Framework 3.0 SP2 x86 is not installed.
[11/07/09,17:38:22] Microsoft .NET Framework 3.0 SP2 x86: [2] Error: Installation failed for component Microsoft .NET Framework 3.0 SP2 x86. MSI returned error code 1603
[11/07/09,17:38:28] WapUI: [2] DepCheck indicates Microsoft .NET Framework 3.0 SP2 x86 is not installed.


Has v3.0 already to be present? I though since v2.0 .NET is cumulative?

How can I find out which .NET version(s) are already installed on my computer?

Richard