If you really wish to find the virus, I might have a good suggestion.
If not already using the world's best antivirus (Avast), install it on a PC connected to the backend of your network (ie, the PC is not behind it's own NAT firewall or something). Disable al Windows firewalls on that PC so the there's nothing blocking the wurm.
Make sure Avast is set to defaults, as the defaults will enable the network shield. The network shield will notify you immediately when a worm or the like is trying to copy itself to that PC. Avast free Home edition is more than enough for this purpose.
You could also setup Avast notifications so Avast will MSN or Mail each and every threat to you. Avast will also tell you what kind of virus/worm is infecting your PC (TRJ, WRM etc where TRJ is a trojan, WRM is a worm etc).
90% of our clients are now running Avast, and such problems are really limited.
The only thing Mikrotik lack, is to send power to your High Sites wirelessly.....