AntiVir has a lot of false positives… But you must be sure. On-line scanning is the best way. http://virusscan.jotti.dhs.org/
If you’re using AntiVir (free version), you don’t have to uninstall avast. You won’t be two residents and the programs won’t be in conflict.
Of course, the Heuristic settings could answer why avast! does not detect that file…
For me, AntiVir detect a lot of ‘macro’ false positives, specially in zip archive files.