Trojan proxy server ??? HELP

rec'd message from symantec "spykiller.exe was not scanned for viruses because it is encrypted. THe container setup.exe was not scanned because it is corrupted  (Symantec decomposer reports DECERR_CHILD_DECRYPT)  if you are able to open it use caution when doin so as it may contain files w/virus's)  Another message that a great deal of spam came out of mycomputer this week, most likely computer is infected w/recent virus & now runs a trojan proxyed server (most disturbing!!!) I have ADWARE and installed MIcroS Anti Adward and anti virus, says nothing is there.  WHAT DO I  DO

thanks so much    Stef