If you are running Avast 6 and are currently finding your favourite/usual sites are being flagged as malware do not worry. This has been caused by a severe fault with the latest definitions update from Avast, which is obviously creating a heap of false positives.
When I got on the computer this morning and started going through my usual procedures I couldn't connect to many sites and it seemed Avast was flagging every site I visited as malware. It even flagged several of the site advisory services included in the Firefox extension LinkExtend as malware.
So I disabled Avast's Web Shield component and then Googled this issue; it seems there are thousands affected.
Here is a temporary workaround until Avast fix the problem permanently:
Right click on the Avast icon in the notification area of the taskbar and select Open avast! user interface
Click on Real Time Shields
[in the left hand navigation panel].
Click on Web Shield
and disable the option to Scan web (HTTP) Traffic