The free online McAfee scanner picked up two files with a Downloader-BCS Trojan but Avast can’t seem to find them.
Search is unable to find the files on my PC either.
McAfee seems to think it’s in C:\documents and settings\all users somewhere
C:\Documents and Settings.…\1d36a3bd-6f5441ad Downloader-BCS
C:.…\game.class-61c915b-56c286bf.class Downloader-BCS
Is McAfee wrong or what?
Don’t know. Upload the file to www.virustotal.com and see what other scanners say.
There is no file to upload. I couldn’t find them with the search function.
However I just used Trend Micro HouseCall (online) and it has removed it. 
This was what I was referring to: http://www.trendmicro.com/vinfo/virusencyclo/default5.asp?VName=JAVA_GIMSH.A
If as suggested it is a JAVA applet (exploit) then I would ensure that your JAV is fully up to date.
Ensure you have the latest version of JRE (JAVA Runtime Environment) because older versions can be vulnerable to malware. First remove All Older Versions From Add/Remove Programs.
Then get the latest update from here http://www.java.com/en/download/index.jsp
Or latest JRE version 6 update 3 http://www.majorgeeks.com/Sun_Java_Runtime_Environment_d4648.html