OK, since this probably affects a lot of users I'll write something similar to what I wrote to Martijn privately:

Try to discover the IP address that is sending this virus (From: is generally not the sender), and then use a tool like e.g. http://www.geektools.com/whois.php to discover the owner of that address (note that you sometimes get an answer covering a whole bunch of IP addresses, e.g. if you tried to locate our server 158.36.132.22, you would discover that Uninett owns the entire 158.36 range (basically most of the addresses within Norway), and you would have further work to do to get a bit closer to the source). Once you find the owner, tell him/her to look for Win32ppr and clean his/her machine if he finds anything.

If you are as lucky as I was, SoBig.F may cease to be a problem after contacting the administrators of those sites.

By the way, the virus Luc describes is SoBig.F, at least those mails with attachments around 100Kb. The good news is that I think I've heard that this virus destroys itself at a set date, but I don't remember when (maybe September 9?).

Set