Ive dumped avast for Avira AntiVir.
Plus over avast:
- More frequent virus def updates
- High detection rate
- Better GUI
- Better program options

Updates are normally daily and the detection rate tops the chart of all the current free anti virus solutions (and is even high ranked in commercial AV detection charts).
Their GUI has been updated since I last checked it out, and its now much better and very easy to use.
There are some very nice program options, my favorte has to be in the real-time scanning options of only scanning data being written to the disk. This allows me to keep the hard drive read performance which is one of the most important things for any fast, well running machine.
download now*Note: Fully uninstall any existing anti virus products before installing a new anti virus solution (this includes doing any needed system restarts!)