Neither of those two shields should significantly slow browsing to the point that there is really noticable differences, or this forum would be flooded with such reports and I don't see them.
There is most certainly something else going on in your system and I'm at a loss as to what that is. First you can't browse with the web shield enabled (and it shouldn't be the windows firewall blocking) and when the browser proxy is manually set to use the web shield and it slows your browsing. To me both of the above point to conflict which is initially blocking the web shield and once the web shield is working there appears to be other conflict slowing loading (possibly double scanning), but I really don't know want that might be as you have tried uninstalling threatfire.
avast 5 is streets ahead of 4.8 the problem is that you aren't seeing this and I really don't know why.
Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?
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You could try a clean reinstall using the latest avast version, 5.0.396 (as the start point, don't know what version you initially installed). I'm taking the precaution of running two uninstall utilities, one specifically for avast 5 and the old one to ensure any remnants of avast4 are still present.
Download the latest version of avast 5.0.396 free antivirus -
http://www.majorgeeks.com/download1968.html?2010-01-29 and save it to your hard disk.
Download the avast5 Uninstall Utility (internal build)
http://public.avast.com/~forejt/aswClear.exe.
Download the avast! 4 Uninstall Utility,
find it here and save it to your HDD.
- 1. Now uninstall (using add remove programs, if you can't do that start from the next step), reboot.
- 2. run the avast! 5 Uninstall Utility, run the avast! 4 Uninstall Utility, reboot. If step 1 failed it may be necessary to run this from safe mode, once complete reboot into normal mode.
- 3. install the latest version, reboot.