Resolving .NET framework 1.1 Installation Error Code 643 with Windows Vista
If you are reading this, you have probably tried just about everything, however, we need to check the basics first. The first thing to do is remove the 1.1 framework using the control panel uninstall. If this is successful, reinstall the 1.1 framework and then SP1 if it was not included in the version you downloaded.
If, like me, during the unistall, the framework is not removed and no notice as to why is displayed, you will need to use a different method to do the uninstall. A .NET removal tool is available, Click here to download. For more information on the tool, you can read the blog entry of Aaron Stebner at Aaron Stebner’s Weblog
Once the framework is removed, proceed with a normal installation.