I've been using the MS Office plug-in Officetab 1.22 for a while, but now Avira Antivir flags it as a virus. Is this so or just a false positive?
i'd say it is the latter, Avira recently (after a definition update) flagged Voipstunt as malicious, it was fixed the next day.
however, you may submit the suspicious file here for further analysis:
leave your email and select "False positive suspicion"
If I were you I will send the file to this web checker that sends your file to ALL the anti virus engines available. Then you can see the results the give.
For me its the best antivirus solution (not doing realtime checks of course).