Spam Advertising / malicious code in my code

This has happened to me twice before. If you check my source code on my blog, you get this –

</body>
</html><div style="display:none"> 

<a href="http://handelsondernemingbejo.nl/topik/page.php?id=4">.50 bewolf</a>
<a href="http://handelsondernemingbejo.nl/topik/page.php?id=16">1 4 to 3 8 m1438</a>
<a href="http://handelsondernemingbejo.nl/topik/page.php?id=17">1 chochran</a>
<a href="http://handelsondernemingbejo.nl/topik/page.php?id=48">11750 huntington station ny address</a>
<a href="http://handelsondernemingbejo.nl/topik/page.php?id=54">122 brookwater drive</a>
<a href="http://handelsondernemingbejo.nl/topik/page.php?id=68">1423 valley view road chaska mn</a>
<a href="http://handelsondernemingbejo.nl/topik/page.php?id=87">1750 e hickory decatur il</a>
<a href="http://handelsondernemingbejo.nl/topik/page.php?id=96">1886 duetz tractor</a>
<a href="http://handelsondernemingbejo.nl/topik/page.php?id=99">18th century music in dressage</a>
<a href="http://handelsondernemingbejo.nl/topik/page.php?id=101">1905 280th st 50511</a>
<a href="http://handelsondernemingbejo.nl/topik/page.php?id=110">1943 battle bastone</a>
<a href="http://handelsondernemingbejo.nl/topik/page.php?id=114">1952 dodge meadowbrook station wagon pictures</a>
<a href="http://handelsondernemingbejo.nl/topik/page.php?id=120">1962 good friday noreaster</a>

and about 300 more lines of that.

Two questions –

1- How do I get rid of it?? I looked at every page on my theme and cant find it anywhere.

2- How did it get there?? This is the third time! The only reason I noticed it the first time was because google sent me an email explaining I was kicked out bcz of malicious code.