It is possible there was a 3rd party exploit via one of the ad slots on the forum. This has happened in the past with this and other forums (not in our control).
I got my first ever trojan infection around the time you got yours. I can't say for sure if it came from EM, but I do recall an EM page "stalling" prior to all hell breaking loose.
I since discovered that Java had a known vulnerability:
Java code-execution vuln exploited in drive-by attack ? The Register - and I seem to recall java starting on loading an EM page, which was odd.
So while cleaning up the system, I also removed Java, until they release an update.