New Avast user. Previously, used Norton AV 2009. Norton AV 2010 had a conflict with Outlook 2003 as installed on my XP computer so when my subscription expired, I looked around for another antivirus program. I presently use Comodo Firewall D+, Sandboxie, FF 3.6, and surf from a limited user account.
I uninstalled Norton through add/remove. Then ran Norton's removal tool. Then manually removed left over files and registry entries after running CCleaner. Then I installed Avast 5.0.
The installation went without a hitch and I have had no problems with Avast or my computer since. The first thing I noticed, was that my computer was noticeably faster running Avast as opposed to Norton. Boot time was much quicker, browsing too. Program loading also faster. And, the problem I had experienced with gmail while using Norton (had to refresh the inbox before I could navigate to other folders) disappeared.
Since I had no previous versions of Avast and completely removed Norton prior to installation, I have experienced no conflicts and am very pleased about the improved performance. For those of you who have had problems with new installs, I suggest the problem is with a dirty system, as opposed to Avast.