I don't know any anti virus software package good enough to tell them apart from each other
AVG. It has won awards, including the VB100, which, from the VB100 site:
In order to display the VB100 logo, an anti-virus product must have demonstrated in our tests that:
* It detects all In the Wild viruses during both on-demand and on-access scanning.
* It generates no false positives when scanning a set of clean files.
The product must fulfil these critera in its default state.
In addition, AVG is efficient enough that you can actually do other things while it's scanning with no noticeable slowdown. I couldn't be happier with it. Oh, and it's free for personal, non-commercial use.