What does Unable to scan: The file is a decompression bomb mean?
Decompression bomb is just something that unpacks to an unusually big amount of data even though it's rather small (i.e. has a high compression ratio, for example). It's nothing to worry about, you are just informed that avast! will not try to unpack the archive (you may not even know that it's an archive, but it seems like it is) because it may take VERY long to process.
(quoted from Igor:
http://forum.avast.com/index.php?topic=15389.msg131213#msg131213)
Decompression bomb is a file that may be rather small, but decompresses to an enormous amount of data (when processed as a packed archive). Such file are not malicious per se, but they may block an antivirus program when it tries to scan them.
This kind of files is rather hard to detect (and avoid) precisely - so, it is possible that there are some false alarms. It's not a big problem in this case, however - the "decompression bomb" announcement actually means something like "The file has a very high, maybe even suspicious, compression ratio and the AV is not going to scan the archive content".
I'd suggest to ignore these files.
But you can change values into avast4.ini file to configure how avast should work with these files.
Click 'Settings' in my signature for more info