Most AVs will report a virus. Trainers work by injecting code into the memory of an executable. Which is a common trait for viruses also. AVs that use heuristics will check for common patterns in code for virus-like patterns, and will detect the ability of the file to inject code. False positives are common. For more information check the FAQ, and Troubleshooting Guide.
[Edited by Sil3nt_Pr0digy, 4/28/2010 3:34:06 PM]
You are correct. And furthermore here is the elusive page that everyone needs to read:
Link