Actually cleaning the file is not going to resolve why you got hacked it will only clean the file (well avast doesn't clean the file just alerts to it, you have to find and strip out the injected code) and not the cause, you need to contact your host, see below.
-- 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.
