OK. Can u tell me why did the autorun idea seem possible to you. Anyway its up to you. IT is not always that virus can only mean hacking. IT can also be transmitted over the pendrive or you might contract infections from the net which becomes hard to remove. And also not your vulnerable position from the security perspective might be exploited by an amateur hacker or a beginner hacker. Frankly wording it Avira free is not enough. Avira aint capable of detecting zero day malawre as avira pro-active its behavioral engine is not present in the free edition. At least switch to Avast OR Avg Free.
From your post i see you are security expert. Thus i would recommend u to try the free Comodo Internetsecurity. Dont worry its light weight.
The autorun idea can work if the files is password protected as when the virus tries to replace it it would need the password. Also changing the attribute of the file to read only may help. Seriously consider a full security suite. Better safe than sorry. My two cents.
This was the same idea my dad had until one day he got a confirmation message on his ICICI bank card about 90,000 transaction at a shopping mall in Banglore.
So its now upto u to decide.