Find out the name of the Trojan- your AV software will tell you this.
Then do a search for the Trojan on your AV vendor's website, or do a
Google search, perhaps also including the word "removal." Different AV
companies may have different names for the same Trojan, so take note of
"aliases."
In addition to the googling mentioned in other posts, you could try a 30
day trial of TrojanHunter, Tauscan, or Trojan Defense Suite 3. All of
those have decent reputations IMHO.
Also there is a free anti-trojan called a-squared (a^2). Kaspersky is
an antivirus that has good trojan detection.