Hallo,
there might be various causes:
1) is the command executed by cron or not? Try to replace it with something trivial, like "echo hello >somelogfile" etc.
2) whis user this crontab belongs to? I suppose it's different user than root (because You stated that avastcmd works fine under root), but then, are you able to run avastcmd manually under that user?
3) when you are sure that the command IS executed, and WORKS normally, then prefix the /usr/bin/avastcmd with "strace -f >your.log 2>&1 /usr/bin/avastcmd .....", and analyse the "your.log" file (or paste it here and we'll have look at it).
Regards,
PC