I'm frequently a person who fills out the original forms on the vendor site, but then doesn't proceed to the download, or to the install. My 4 biggest turnoffs are:

1. When it comes time to do the download, the site doesn't respond to my browser, is extremely slow, demands security be turned off, or shows errors. This happens very often.

2. If during the install, the registration / activation requires an analog phone-line, I'll cancel the install and remove the software.

3. If during the install, the package insists that I turn off my virus checkers, its gone.

4. After the install, if there isn't a de-install available, I revert the system.