is there any other security program installed on your comp?
Not then nor now. What I used was Revo Uninstaller Pro and I ended up in the dead man's loop. You would think there would be a way to force the installation or at least a way for us to remove the trail that is keeping Avast from installing telling me I need to reboot first yet I had rebooted at least 20 times as I was trying various things.
Umm, Revo should never be used for uninstalling security programs. Which is why only the vendor's final uninstall tools should be used. Link provided above will suit your purposes just fine.
Want to bet? As I said nothing was on my system. No lingering (orphan) registry entries, no folders, nothing I could latch on to so the uninstaller would work. Just clicking it, as I also said, did not work either.
Considering Revo brought up your uninstaller all I can say is this is an issue with a sticky program with no regards to the end user. Hell, even AVG, which I despise, has a force option for rare cases like this but yours had nothing and I had to reinstall an image which did not have Avast on it because your software, regardless of the reason, forced me to take that drastic of a measure.