This problem drives me insane
1. In advanced setup in computer protection disable stealth mode
2. Make sure you are full administrator from USERS OU , if not create a a new one by copying the administrator user as a template (administrator on modern OS is disabled and in SBS versions the administrator does not have the correct permissions to delete ESET services)
3 . Log on as new administrator or if already fully fledged , disable all eset services, if you get access denied you may not successfully uninstall (about 80% success rate)
4 Reboot server
5 Log on as administrator again and make sure no ESET services are running, Right Click the ESET unistall follow from programs (server 2012 you will to press CTRL + tab on metro screen to see it) and run as administrator (server 2003 take the tick out of restricted access)
6 Should install fairly quickly though Mail security can take a couple minutes longer.
7 If all else fails use the eset uninstaller , though I have known eset to come back after using this method