Avast problems, Windows Vista?

I'm the Vista user, and installing avast 4.8 free for virus protection.

One day, my laptop faced with problems, so by using restore point, I restore my laptop back to 1 week before. Generally my laptop work well after restoration, except my avast! antivirus.

I believe my avast! may be corrupt because the restoration. I tried to uninstall it, but I fail. I also tried to delete the avast files in Program File, but my laptop prevent it.

Any idea how to uninstall my old avast?

It give me problem because when I want install the new Kaspersky antivirus, the warning will ask me to uninstall my Avast first, although my avast actually not work.