Thanks, I've already solved the issue. It was no virus at all, but a code to hide e-mail address from bots. Avast apparently searches for occurence of eval() functions and decides that there's a trojan if it finds it.
This was the script causing the error. For anyone with the same trouble, search your files for "eval".
<script type="text/javascript">
//<![CDATA[
<!--
var x="function f(x){var i,o=\"\",ol=x.length,l=ol;while(x.charCodeAt(l/13)!" +
"=48){try{x+=x;l+=l;}catch(e){}}for(i=l-1;i>=0;i--){o+=x.charAt(i);}return o" +
".substr(0,ol);}f(\")36,\\\"610\\\\400\\\\010\\\\330\\\\320\\\\610\\\\N100\\" +
"\\120\\\\ADI700\\\\IUC\\\\\\\\VBI@Pplk}[{owdww:r`eu771\\\\0/P)V4mkrlp# ]m12" +
"0\\\\630\\\\R230\\\\010\\\\430\\\\100\\\\500\\\\720\\\\630\\\\520\\\\300\\\\"+
"530\\\\300\\\\600\\\\610\\\\.410\\\\230\\\\400\\\\130\\\\010\\\\n\\\\I700\\" +
"\\720\\\\020\\\\600\\\\220\\\\[710\\\\+24=6x500\\\\e13'<s3mrg !)?#;?i2+!.7\\"+
"\"\\\\/[\\\"(f};o nruter};))++y(^)i(tAedoCrahc.x(edoCrahCmorf.gnirtS=+o;721" +
"=%y{)++i;l<i;0=i(rof;htgnel.x=l,\\\"\\\"=o,i rav{)y,x(f noitcnuf\")" ;
while(x=eval(x));
//-->
//]]>