Many users of Avast! Anti-Virus are reporting that Avast! keeps on detecting trojans or other malware every time you would go to any website, (e.g. facebook and twitter) It appears that Avast! reports the threat as a “HTML Script inf” and one user reported that Avast! found 324 infected files, but the issue is that Avast! pushed a bad virus definition file, and Avast! will fix that problem with an update. Hopefully that will fix the problem.
(Update 4/11/11 at 6:56 PM: Avast! issue resolved.)