I recently installed a fresh version of the latest official release along with the Costa Nature theme. I made no mods to the code, I made a small change to the template (different header pic), and only installed official plugins from this site.
Now I am getting some messages from a couple of end users that their virus scanners are warning of malware on the site. Does anyone have a recommendation on what I should look at to address this:

The message in the photo says:
Anyway, it says:
File name htto: //wertionase.com/cache/pdf.php
Malware name: JS-Agent-BQ(Expl)
Malware Type: Exploit
When I run the site in Windows XP using Avast, I do not get any warnings, but some Avast and Norton users are. I'm stumped.
Thank you in advance for any pointers.
Allison