The first error is not critical: "Could not update environment variable 'Path'. Verify that you have sufficient privileges to modify environment variables."

(I can always change the Path variable myself.)

Click "Continue" to disregard this error and continue the installation.

The second error is critical: "An error occurred during the installation of assembly 'Microsoft.VC80.CRT,type="win32",version="8.0.50727.42,publicKeyToken="1fc8b3b9a1e18e3b",processorArchitecture="amd64"'. Please refer to Help and Support for more information."
Comments:
[arif] Do you have admin privileges on your vista machine?

[kenneth] Thanks for your help. I'm all set now--got it to install. Although I am logged into the administrator for my Vista machine, that doesn't execute a file in Administrator privilege mode when double clicking on the MSI file, nor does executing the msi from a command prompt in Administrator privilege mode. It seem that I have to run "msiexec /a graphviz-2.24.msi" from a command prompt. It would be nice to add a note to http://graphviz.org/Download_windows.php to do a "msiexec /a ..." from a shell.

[arif] I have heard that problem before. I think it is a good idea to put a note on windows download page

Vista users: Due to Vista's paranoid behavior , To install Graphviz you may need to run "msiexec /a graphviz-x.xx.msi from a console window rather than double clicking on it.
Owner: arif
Status: Fixed (21 Oct 2009)