"You're right about legacy apploications but things need to be seriously old to cause that sort of problem."
Sage Payroll, current versions. So not a small company or a niche product, and not really a legacy app. It STILL doesn't handle UAC properly so to install an update for it you need to be logged in as an admin user. If you attempt to install the update as a normal user and enter the admin credentials via UAC you find part way through the install it breaks out of the elevated security context and back to the user context... which doesn't have permission to do the update so the update crashes and often hoses your Sage Payroll installation.