-1603 fatal error during installation

In trying to install itunes 6 I saw one error (can't remember which) and then the need for Quicktime 7.0.3--finally did the stand alone download as per this forum. Now I get "-1603 fatal error during installation. Consult Windows Installer Help (MSI.chm) or MSDN for more information".

Naturally Quicktime 7.0.3 won't finish installing so I can't get itunes 6 to work (what a pain!). Any ideas? Thx.