Hi guys 
I’m new one here and I have a problem with avast
My avast didn’t boot with Windows(I have the XP SP3) then I tried to run it on desktop icon but nothing happens. I tried scanning with SUPERAntiSpyware and IObit Malware Fighter(I don’t use it very much because I have Avast ) but nothing was found. Then I went to reinstall but if failed, he deleted some leftover files in registry but all files in Avast folder are still inside. Then I opened task manager and tried to terminate AvastUI.exe and avast.setup(I don’t know why it is running anyway) but access is denied. I wanted to terminate them so I can just delete avast folder. What should I do? Try scanning in safe mode?
Hi there you will need to do a clean install
Lets reinstall Avast
Download Uninstall Utility to your Desktop.
Download the correct version of Avast
Avast Free
Avast Pro
Avast Internet Security
Avast Premier
Disconnect from the net
Uninstall Avast via control panel
[]Run aswClear
[]It will offer to reboot to safe mode … Accept that
https://dl.dropbox.com/u/73555776/aswclear.JPG
[*]Once it has rebooted to safe mode
[*]In the Select Product to Uninstall dropdown choose the version of Avast that is on your system.
[*]Press Uninstall
[*]Once complete reboot your system to Normal Mode
[*]Reinstall Avast
Thanks, it was simple and efficient
Avast the best 
You are welcome 
I’m having the exact same problem on Win 7 x64 now. Followed the steps exactly, didn’t work.
Trying to install v8, not sure how to find exact version now as it was downloaded a few days ago.
History: downloaded, unplugged network cable, uninstalled old version, rebooted. Installed new version, install ran fine, got Access denied error. Googled, found out about blocked files in Win7, discovered setup file was blocked. Uninstalled, rebooted again, unblocked setup file, installed. Same Access denied.
Interestingly, if you right-click the shortcut on desktop (icon is a generic icon, Avast icon fails to load for AvastUI), then properties, then try to open Compatibility tab, it gives Target not valid error: “the name “C:.…\AvastUI.exe” specified in the target box is not valid. make sure the path and file name are correct” even though I used default (even express) install settings.
Any help appreciated.
PS previously, AVG was installed by the IT shop. We brought it into the work environment where Avast is used. AVG was uninstalled, but not completely removed using a removal tool, before attempting to install Avast the first few times. Latest time, we used the AVG removal tool as well, still failed.
PPS Avast works in Safe Mode and Safe Mode with Networking.
PPPS for one of the installations, I changed the owner of the Avast folder in Program Files (right-click, Properties, Security, Edit, Change Owner or something), to see if it wuold help. It helped the administrator, but if it went to a user’s account (which it successfully did only once), it did not help.
Problem: Threatfire
Solution: run services.msc as domain level admin, set startup type for Threatfire to disabled. Exit Threatfire (CTRL + RIGHT CLICK → EXIT). msconfig → Startup → untick Threatfire (couldn’t be bothered with GUI for now).
Then reboot, unblock Avast setup file, install.
Reboot, then wait for Avast to run. Re-enable Threatfire service, start threatfire service, start threatfire. Both work fine again side by side as they have done for YEARS. The only hint (to a novice) that Threatfire was the problem was 1 threat detected at the end of the install. I permanently allowed the process to continue, and thought nothing more of it. Seems that was the killer…
PERMANENT WORKAROUND:
Set the startup type for ThreatFire as Automatic (Delayed), and make sure Avast service is set to Automatic. Then it will allow Avast to run first, and Threatfire will quite happily startup later (about 5s depending on your setup) and run as well.
Please remove Threatfire. PC Tools has discontinued it so it is no longer being supported.
You should run the AVG uninstaller, you can find it here http://singularlabs.com/uninstallers/security-software/
You should also uninstall Threatfire as running two resident anti-virus programs can and will create conflicts.
Having two a/v’s is just asking for trouble. See http://www.bleepingcomputer.com/forums/t/260844/two-anti-virus/#entry1441638
Then do a clean install of avast.
Thanks guys for the info. Back in the day Threatfire and Avast! performed two different functions, or so I was led to believe. Now it seems Avast has covered both, and as you say Threatfire has ended.
I did use the uninstaller aswclear.exe (assume that’s the one, having the bandwidth problem here like other users so unwilling to check). Avast! is working now so will just remove Threatfire.
Thanks again for confirming.