With a little trial and error I've managed to set up pFirefox to use my install of pClamWin AV to automatically scan downloads. I am using the Download Statusbar 0.9.4.1 extention and used the example from the ClamWin website for enabling virus scans in Winzip. It required one change to keep pClamWin AV from choking on the spaces in the filename.
From the ClamWin website (http://www.clamwin.com/content/view/117/89/):
Scan Program: C:\Program Files\ClamWin\bin\ClamWin.exe
Parameters: --mode=scanner --path="%d" --close
In the Virus Scan tab of the extension options:
Anti-Virus program location: G:\Portable Apps\PortableClamWin\PortableClamWin.exe
Arguments: --mode=scanner --path=%1 --close
I haven't tried this on other computers yet to see if the path will play relatively, but it's something worth trying. It's also not particulary elegant. It downloads and saves the file before it scans it, and if you have told it to open the file then it is going to open the file. pClamWin AV is configured only to warn and not to quarintine the file. Fortunately pFirefox only allows you to save executables.
It catches all four of the files from http://www.eicar.org/anti_virus_test_file.htm
Hope this is useful to somebody.
jpe