1. 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
2. By examining 1) the reason given by avast! for not being able to scan the files, 2) the location of the files, you can get an idea of what program they relate to. You may need to expand the column headings to see all the text.
Files that can't be scanned are just that, not an indication they are suspicious/infected, just unable to be scanned.