avast doesn't get rid of it, that is down to you or your host to remove the malicious code which has been inserted, your site has been hacked.
All that avast does is detect and alert to the detection on your site and the option is abort connection, which just stops the infected item being downloaded to your browser cache.
What is your web site URL,
change the URL from http to hXXp or www to wXw, to break the link and avoid accidental exposure to suspect sites, thanks.
-- Every 3.6 seconds a website is infected
http://forum.avast.com/index.php?topic=47096.msg396648#msg396648.
-- HACKED SITES - This is commonly down to old content management software being vulnerable, see this example of a HOSTs response to a hacked site.
We have patched up the server and we found a weakness in PHP which was helping aid the compromise of some domains. We updated it, and changed some default settings to help prevent these coding compromises. The weaknesses were not server wide but rather just made it easier on a hacker to compromise individual end user accounts.
I suggest the following clean up procedure for both your accounts:
1. check all index pages for any signs of java script injected into their coding. On windows servers check any "default.aspx" or
"default.cfm" pages as those are popular targets too.
2. Remove any "rouge" files or php scripts uploaded by the hackers into your account. Such scripts allowed them to make account wide
changes, spam through your account, or spread their own .htaccess files through all of your domains in that end user.
3. Check all .htaccess files, as hackers like to load re-directs into them.
4. Change all passwords for that end user account. The cp password, the ftp password, and any ftp sub accounts. Make sure to use a
"strong" password which includes upper case, lower case, numbers and NO COMPLETE WORDS OR NAMES!
This coupled with our server side changes should prevent any resurfacing of the hackers efforts. In some cases you may still have coding which allows for injection. All user input fields hidden or not should be hard coded, filtered, and sanitized before being handed off to php or a database which will prevent coding characters from being submitted and run through your software.
Also see, Tips for Cleaning & Securing Your Website,
http://www.stopbadware.org/home/security.