Problem description: When attempting to install Dawn of War, an error message is generated, stating that "The installation source for this product is not available".
Cause: This can happen when Dawn of War was previously installed on the system. If the application was not removed correctly before attempting another install, this error may pop up.
Solution: Have a look at this article on microsoft.com. Download the tool linked there and use it to find and remove previous Dawn of War installations.
Note: Even though the article states that this solution only applies to Microsoft Office, this can in fact happen with other applications like Dawn of War as well.