Automated Process/Virus?

Hi, I don't know whether this is the right place so my apologies. I recently noticed that the banner on the top of my site was missing. I re-edited the page and corrected it. However, it was then removed again. I noticed that more and more (but not all) pages are having this banner removed.

This is what is happening:
<TR>
<a href="URL"><img border="0" src="Image Path" width="720" height="200"></a></td></tr>

is beign turned into:
<TR>
<a href="URL"> </a></td></tr>

There's just a long line of spaces where the image path was.

This is happening to some directories that I've not edited for over 8 months and there's no evidence on my FTP records that these have been modified.

I was wondering whether this is some sort of automated process that I need to remove. I've never attempted to install an automated process. I was also wondering if it could be some sort of virus. Any advice is welcome. Thanks,

Daniel Gaunt.

Not likely to be an automated process or a virus. It's quite likely an ad filtering program you've got running.

--Simon