I got a case here where I couldnt install avast 4.8 on a Vista (SP1) PC. It can install the home edition but not the Pro edition. I manage to install it on other PC/Laptops with Vista but not on this particular PC. Need help in resolving this
What other security applications are/have been installed on the PC? Could be interference from leftovers of something, say, Norton, or McAfee, for example.
You could also try leaving the Home edition installed, and using the “upgrade” link available from left clicking the tray icon to get pro. The programs are identical, inserting the purchased license key will activate the Pro features in a Home installation.
Welcome to the forum. Just follow the suggestions given by Tarq57 and tech.
I recommend, if possible, to update Vista to the latest Service pack (SP2) as well. It’s more of a security precaution as the SP2 and other updates fix security weaknesses in the OS.