I experienced this same issue upgrading from workstation 7.1.x to 8.x due to recovering a Windows 7 machine from a 2 year old backup. WoodyZ’s pointer to manual removal instructions did work for me but I found a much simpler way if you still have the previous version installation application. If you do, then launch that installer, and select repair installation. You may be instructed to reboot. Then launch the previous workstation version installation app again and select the remove installation option. Select this should cleanly remove the previous install. You will be asked to reboot. The new workstation version should now load without issue. Hope this helps to those that may come across this issue.
