There might be an archive the avast! has problems with (crashes when scanning it). As Technical suggested, please turn on the creation of the report file, let OK files be included there and run the scan again.
After avast! disappears, check the end of the report file - the "troublesome" file is likely to be close to the end (close in the sense that this particular file will probably not be written in the report, but the previous one will be the last line, so it shouldn't be hard to guess). We would certainly like to have this file - if it really causes problems to avast! - so that we could fix the problem.
Also, what is the exact build of avast! that you use? 4.6.623?