How do i delete Avast antivirus from my computer? it won't let me D:!please help!?

if anybody knows, I could really use some help. I had download a free trail (but now that the trail is up, my computer started filing errors) so i tried to uninstall it by going to my computers control panel, i clicked uninstall, and deleted the folder and the shortcuts...but no matter how much i try i can't delete the permanent avast file (on my local disk files) , it says i need permission from the administrator, but I'm the administrator because my computer only has one user. WHAT CAN i do???? ...i also tried loging on to my avast account but really have no idea what I can do.... if necessary my laptop is an Acer, windows vista 7, 64 bit operation system help would be really appreciated :)!