Yes sometimes it happend that u cannot uninstall something, when that happens with an antivirus, it sure is a big trouble. This happens becauz of some clashes with the OS and AV! so i recomend to use some mcafee removal tool or just kill all th process and delete the folder along with registry entries and startupz also dont forget to delete the services(kill them before this)..
Or in the last thing is to FORMAT ur OS drive..